I have a 100% cotton t-shirt.  How do I get it to shrink?

If the T-shirt has a "sanforized" label, it should not shrink more than 1%,
but you can try by washing and drying it at a high temperature.
If it does not have a sanforized label, this will shrink it, maybe
more than you want, so just wash it hot and dry by hanging  - not in
the dryer.  Repeat, if not shrunk enough, using the dryer,
but I take no responsibility if it is then too small.  ;)

Odds are that it is sanforized.

After wash, put it in a drier on High Heat. But it won't work if the
T-shirt is made of pre-shrunk cotton.

Cotton is a natural vegetable fibre which is obtained from the seed
pod of the cotton plant. The cotton fabrics which are now available in
the market are blended with some other fibers like polyester, silk
etc. So the shrinkage of the fabric depends upon different fibers
which constitute the fabric. Generally ,the cotton fabrics tend to
contract when wet and tend to shrink as they dry. Desizing or
shrinking the fabrics is aimed at degrading the starches to more
soluble products such as dextrin. The following are the steps to
shrink a 100% cotton fabric.