Welp let's see here.
victoryfactory.com has really nice ones that aren't too expensive. I got my huge 18x22 one for $18 and there are also smaller, cheaper ones. If you want to do multiple colors for one print, I would recommend getting more than one screen. (It's not completely necessary, but it makes things easier/quicker)
-Ink $5-7 per color
-Scoop Coater/Trough $8-10
-Screen Cleaner $6
And then depending on whether you want to use drawing fluid or photo emulsion, you would get different products.
-Drawing Fluid $5
-Screen Filler $18
-Photoflood lightbulb $6
(This is optional. You could also use the sun as a light source, or a normal light bulb. But keep in mind: the sun is a VERY inconsistent light source and could easily mess up your print/emulsion and a normal bulb takes an hour+ to expose and it's not very efficient)
So roughly anywhere between $70-100 in total.
You can find all these things at either victoryfactory.com, utrecht.com, or dickblick.com
OR, you could just """borrow""" some of the supplies from your art teacher, like I did. ;)
Silk screening squeezes ink through a fine fabric mesh, through a stencil and presses the ink onto the printed surface.
Iron on transfers have ink printed on a piece of paper with a plastic coating. When the tansfer paper is laid, printed side down and a hot iron is applied to the back of the paper, the plastic layer melts slightly and embeds itself into the fabric of the printed surface. Since the ink is between the plastic and the fabric, it is trapped and colors the fabric.
After that, the biggest difference is that the iron on transfers are very fragile and will not survive may washings. They actually show cracks and fading after the first laundering. A good silk screen, however will actually outlast the shirt itself.
Iron on transfers can be as cheap as one or two dollars apiece. A good, multi colored silk screen can cost a couple of hundred dollars for the first one, and just a few dollars for subsequent copies.