I make my own tie-dyed tee shirts (because basically I'm still a hippy at heart!). I do the following:
Get a new but washed white tee shirt, some dye of your preferred colour and some string.
Lay the tee shirt out and then just 'grab' a part of the shirt so that it forms a bunch in your hand. Tie a piece of string very tightly around the base of the bunch of material. Do this as often and in as many places as pleases you, just make sure the string is very tightly tied off.
Make up the dye as per instructions on packet and completely immerse the shirt in the dye and leave it for as long as instructions say.
Remove the tee shirt, cut the string and wash the tee shirt to remove excess dye.
This will give you the classic 70s look plain coloured tee shirt with white circles/spirals on it.
There are some more complicated methods using multiple colours and wax relief that are available on the net if you do a search.