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Theme
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.......... The design MUST HAVE ONLY 2 COLORS; BLACK AND WHITE
this journal 
-where do you even get started?
what is the cheapest way to screen print?
OR what is the best website that does this?
how do you gauge how much to sell things for?
any other information/tips are awesome!
i'm doing some looking into it myself right now since my aunt and i had a serious conversation about setting things up for me to get some extra money since i'm in the poor house.
and so you guys know i wasn't lyin about drawing..
NEXT KIRIBAN 444,444!!! 
Zeal 
also, i've found that online print companies arent all that wonderful. your art is kinda like mine colour-wise, and printers have a hard time copying the vibrant shades. i've had a lot of prints go from teal on my computer, to navy blue on the shirt...it's best to find a local print shop; that way you can talk to the printer, and give him/her all the colour info. they will usually do a test print free of charge (shop depending, of course), so you can make sure it turns out the way you want.
hopefully i didn't just repeat what everyone else said, haha, and good luck. c:
the site works really well for large simple images but a lot of the people i work with refuse to use it cause sometimes you'll get back shirts with pixelated or otherwise blurry and messed up images.
the way they do it is more like a computer printer working on the shirt not actual screen printing
wolfspectre gave some really good advice. just make sure if you do it from home start out with a premade screen until you feel confident enough, keep your emulsion refridgerated when not in use and never expose it to light unless you're burning it, if it gets on your clothes and you walk out into sunlight it will be on there forever if you don't get it in the wash in a couple of minutes, and double check your inks there are some that are meant to be used on fabric and will work and look good if heat set properly but a lot of the ink out there is meant for paper and will not work correctly on fabric, also if making or buying screens check first to make sure the mesh is a high enough count to work on what you're doing. you want really fine mesh that lets a lot of ink through for fabric
printing homemade shirts usually sells better and gets more attention in the minicomic and indie scene but it's starting to catch on a little more. i'd suggest unless you're doing a one color design go through a printer first since it's easier and more consistent with the print unless you want to do limited runs or special editions
I go to the embroidery shop/factory which is nearby, and they have a printer .. thing o_o;;
transfer printer i think it is.. i print my design out on the suitable paper (one side acts like a glue) and it is melted onto the t-shirt then when you peel it off your left with your design 8D its simple and anyone can do this technique.. depending on how much access you have to the equiptment, im lucky to live nearby to one.. i do it myself instead of the company doing it for me and it saves me a hell of a lot of money and i get it done in one day wheras it takes them from 1-2 weeks D:
Price wise.. to make 40 t-shirt it costs us maybe £80.. i think its like £1.30 to make each t-shirt then you have VAT and electric so it adds on..
I sell mine for £10 each, so we make laaaarge profit XDDD
Sorry if my information dosent help much ive only been doing this since last october ..(and im only 14) XDD
But.. good luck! <3
From what I've learned, most places can do regular t-shirts for $6-$10 a shirt (best low prices, some can charge more than that) but you usually have to get at least 35 or more of one design. However it can be on any size and color shirt, so you could, say, put one design on half blue and half red shirts of all different sizes.
You're also looking at a color limit of 6 colors max, and screen fees for each color in a design (usually $15-$20+ each) but the fees are for set up only, so you only have to pay them once per design.
As for what price to sell them, it can depend on the design. Like single-color designs are less likely to sell for as much as multiple-color designs, but if it's cute/catchy/popular enough it wouldn't be a problem. The usual going price for a silkscreen shirt is $15-$20.