The standard tent size is 10' x 10'. Whatever you buy, get that size.
For a first-time crafter, I'd start with an EZ-Up brand tent. I strongly, strongly, strongly, recommend you stick with that brand name. I got mine a couple of years ago at Sam's for about $150. You can also order them from EZ-Up on the web... For that price you get the frame, the nylon top and four walls that tie onto the frame and then zip together. It is sturdy enough for one-day shows and the occasional multi-day show (where you'll use the walls at night, etc) in decent weather. These tents are the easiest to erect and easiest to bring down. Unfortunately, "easy up means easy down". If you get inclement weather, high winds, etc, this tent may not fare as well as others mentioned below.
Now, note I made a pretty strong recommendation for that particular brand. Why? I've seen at least 6 of the Ez-Up knock-offs (and you can get them at sporting goods stores, E-Bay, etc for about $100) in one or more of the aluminum frame struts just snapped in two when it was being put up or being taken down. My friend bought one on E-Bay and the first time she used it, one of these metal struts broke. Can she still use it? Yes...sort of..but she'll need to splint it and 'mess' with it every time she sets it up or takes it down. The same thing happened with my friend Julie. She bought a dozen of these at a good price from a discount sporting goods store...with the identical results.
My E-Z up is a couple of years old, has had hard use and is still going strong...well worth the extra $50 I paid for it.
FESTIVAL TENT - LONG-TERM OPTION
Now, after you've used your E-Z up and you start doing serious shows, multi-day shows and inclement weather shows, what do you do? This is where the serious "festival tents" come into play. These are manufactured by several makers (mine is a Craft Hut) and cost around $1000 for the frame, top and four sides.
What do you get for that? A much sturdier frame (which is also harder than an EZ-Up to put together/take down), much, much sturdier vinyl..probably five times as thick and waterproof an the EZ-Up and uber-strong nylon zippers that take an extreme amount of punishment. Weighted down, these things will withstand gale-force wind, horrific rain/lightning, etc. Note I said "weighted down"...something you want to do with all tents...see below. With Festival tents you also get many more options, since these are generally custom-sewn. Options include awnings on 3-4 sides; matching banners, colors other than white, vented skylights, stabilizer bars, screen panels, alternate sizes (10' x 20', etc). I've used these tents that have been in use for 10-15 years and still going strong. I've only had my Craft Hut for a year and so cannot speak to its' longetivity..but I get glowing recommendations from other artists.
OTHER IMPORTANT CONSIDERATIONS
Weights - Regardless of the kind of tent you use, weigh it down. to withstand wind and rain! The smart shows require weights. How? I purchased a long piece of 4-inch plastic PVC pipe, four PVC end caps, some PVC primer and PVC glue, four loooong eye bolts, some strong nylon webbing and two bags of quick-setting cement...all from Home Depot. I cut the PVC pipe into four equal lenghts and glue on the caps with the primer and glue. Once those are dry (20 minutes, maximum) I mixed up the cement and poured into each of the four pipes, standing them on their ends. Before the cement starts to set, I insert the looooong eye bolts into the end of each of my weights. Once dry, these weigh about 55 lbs apiece...generally the required minimum is 35 or so pounds of weight per corner. I then loop the nylon webbing through each eye bolt and hang one on each corner..and have withstood significant wind and rain this past year. See the picture to the left: My Craft hut, minus sidewalls and with my weights..gotta paint them pretty though!
Others use big barrels of water, spiral tent stakes (which are preferable for dirt/grass, but what if you're setting up on asphalt or concrete?) or 25-50 pound free weights attached to the tent legs.
Fire Retardance - some shows will require you to produce a certificant attesting to the fact that your tent is made of fire retardent material. I don't think EZ-Ups are. My Craft Hut came with such a certificate without my asking for it.
Keeping It Clean and Dry - Both options will deteriorate if you store the tents wet, allowing mold and mildew to form. If you break down in heavy dew or rain, set it up when you get home and allow it to dry thoroughly. If it gets mold, find out what the manufacturer recommends to remove/kill the live mold.
So there we have it....you now know about at least two options for tents and then a little bit about taking care of it and making it work for you. Good luck out there!