For step one toward hijabness... (THIS IS NOT HIJAB, JUST A STEP IN A MORE MODEST DIRECTION INSHAALLAH)
The first thing to do is to get yourself use to wearing something that covers all of your body. It is an adjustment for the way your body cools and heats itself so it is good to get this step going while it is still cool/cold outside! This way you don't feel overheated when you first start covering all parts of yourself.
Also at this point, you can start wearing looser t-shirts and cardigans (instead of just sleeves under a t-shirt).
Best place for cheap, loose t-shirts? Brother/Uncle/Dad's closet! I'm telling you, your dad probably has some really sweet t-shirts from the 70s/80s. Go look for 'em!