That's not the greatest way to ask the question, but new costumers are always asking for suggestions on sewing machines. The best machine for you might just also be the best machine for your budget.
If at any point you find yourself lost in this document, you might want to check out Sewing Machine Buzzwords.
Unless you really need fancy machine embroidery capabilities, you only need a few things from a machine
You need easy set-up and adjustment
You need a good basic straight-stitch
You need a good basic zig-zag stitch
You need a free-arm
An easy-to-use button hole system is a bonus
Easy threading and adjustment is vital. If you're doing costuming, you'll find yourself using a wide variety of threads, and often changing threads several times within a single project. If your machine is easy to thread and the thread tensions are easy to adjust correctly, you'll save a lot of time and effort. Decorative threads are particularly fussy with regards to threading and tension, so keep that in mind
A good basic straight-stitch that's easy to reverse when starting and finishing seams is vital. Most of what you do is going to involve straight-stitching, so a good stitch when you adjust the tension and stitch-length correctly is the most important feature your sewing machine can have.
A good basic zig-zag stitch is vital. Fancy stretch stitches are great and for some materials and projects necessary, but a good basic zig-zag will function well when working with many stretch fabrics. A three-step zig-zag (or "tricot") stitch (where each pass of the zig-zag is made up of three short stitches in the same direction before reversing) is particularly valuable for working with and mending most stretch materials.
A free-arm is vital for beginning costumers. It makes sewing cuffs, armsceyes, collars and certain types of complex curves much easier. As you become more experienced you will probably learn folding and bunching tricks that allow you to sew such items on a flat surface, but having a free-arm makes it much easier.
An easy-to-use button hole system is nice to have. Hand-worked button holes and bound button holes are nice and button-loops are easy, but sometimes a simple, flat buttonhole is the quick answer to a problem. 1-step buttonhole units are very slick and easy; the machine does the work for you. 2-step and 4-step buttonholes aren't much more complicated, though.
Everything else is gravy. "Electronic" speed control is nice and delivers full power at any speed. Stretch stitches can allow you to work with fussy material. "Needle up/down" control is nice, but you can attain the same level of control by hand. Automatic thread clippers are cute, but scissors can do the same thing. Automatic or "in-place" bobbin filling is slick, but not any better in the end than filling the bobbin on a winder. Automatic tension control often doesn't work well.
From the early days when chain-stitch machines were first invented, sewing machines continued to improve and add new features. Chain-stitch was replaced by lock-stitch, zig-zag was introduced, electric power replaced foot power. From the 1940's through the early 60's, European manufacturers such as Tavaro (Elna), Pfaff, Husqvarna (Viking) and Necchi kept leap-frogging one another with new features. Elna introduced the first successful home free-arm machine. Necchi introduced the first home zig-zag machine. Elna countered with the first machine to use interchangable cams to control stitching. Each came up with new features to make them stand out from the others.
Then it all started going downhill.
When Sears & Roebuck started importing inexpensive Japanese sewing machines to replace the more expensive European and American machines in their product line, the cost-saving measures started to kick in. Metal parts were replaced with plastic or nylon parts (the advertising slogan "no plastic gears" often indicated not metal but nylon gears were being used). Rotary bobbin shuttles (which tend to produce a more evenly-tensioned stitch than an oscillating shuttle) appeared only on the most expensive machines. Removable cams made way for internal "cam stacks" that couldn't be replaced by the customer when they failed. Sewing machines got lighter and cheaper, but at the expense of longevity.
Sewing machine companies could no longer afford to manufacture in Europe or America and eventually moved all their production to Asia. In some cases, all that was left of venerable brands was the name, sold to the highest bidder.
There was innovation in the 60's and the 70's, particularly as computer control was adapted, but little really improved a machine's ability to do the basics, and rarely improved reliability. The Singer Futura II 920, considered by many to be the last good sewing machine Singer ever made, was preceeded by the Singer Futura 900, a lemon if there ever was one.
I was getting to that.
Often the best sewing machine for your dollar is a used machine. For the price of an entry-level piece of junk from Wal-Mart, you may be able to find a primo top-of-the-line machine that's a few decades old and will sew a better line.
First you need to find a shop. While it's possible to find great machines cheap at rummage sale or on eBay, that's not the best way to go for your first machine.
Find a sewing machine shop (not a fabric store, not a Wal-Mart) that takes in trade-ins. Find a shop that takes in a lot of trade-ins. If they take in a lot of trade-ins, chances are they have to price them cheap to move them out so there's space for more. Make sure the repair department has a good reputation; chances are they're going to do a tune-up on the machine before they price it and put it on the shelf. If you're really lucky, the repair tech is also the owner.
Go shopping. Take somebody who sews with you. Try out machines.
Bring your own fabric. Those little strips of pink cotton with so much sizing they’re like cardboard they give you in the store are useless.
Try threading the machine. Try it a few times.
Try changing needles. Make sure you know how they fit.
Try the basic stitches: Straight stitch and zig-zag, and try the back-tack function. If you plan to work with stretch fabric, try the stretch stitches.
Try the button hole process.
You want brand recommendations?
I'm partial to the European machines of the 50's and early 60's. The American manufacturers made excellent machines but were often well behind in features. You would be hard-pressed to find an American free-arm machine of that era. Japanese machines of the time just didn't compare in features or quality.
These old machines will work well on all but the lightest, flimsiest fabrics and will stitch through the heaviest materials you can slip under the presser foot.
Pfaff, Elna and Viking all made excellent machines at the time, and many of them are still running. All are machines that you're not likely to outgrow as your skills improve and you start to do more demanding projects. Pfaff's 300 series automatics (332, 332-260, 360 and 362) are excellent machines, basically scaled-down versions of their industrial models. Elna's "Supermatic" series is sturdy and offers many unique features (including a surprising level of portability). Viking's venerable "Model 21" and their 2000 and 6000 series "Colormatic" machines are real workhorses that feature a 5-to-1 step-down transmission that makes both slow and heavy work much easier.
I'm not as big a fan of Necchi. Their machines, while advanced, had design issues; when they broke, they broke. If the internal cam stack went, it was all over. Necchi did make a a series of machines that used interchangable cams (the "Nova" and "Supernova" series), but these were primarily flat machines; free-arm machines in these series are very rare.