Greekgear.com, fratrow.com and sororityspecialties.com let you order just letters if you see something you like. The plus side of getting letters from one of these sources is that the letters come with an iron-on backing so you can iron the letters in place and sew them on to keep them there without using pins. I have made 3 shirts and a tote bag this way.
Here is a website that has stencils for making your own letter shirts. If you want to start completely from scratch, pick out your front and back letter fabrics and get enough iron-on interfacing for each letter twice (front and back). Iron the interfacing onto the fabric, and then cut out your letters. I'm not sure if iron-on interfacing is double-sided or single sided, so if you can, iron the layers together. The iron-on interfacing will help the letters be sitffer and prevent fraying. (Never put your letters in the dryer)
I wouldn't sew letters on by hand; you'll get a much more secure stitch if you do it on a sewing machine, so I'd ask someone to teach me how or ask someone to do the stitching for me.