When I was in Middle school, my mom bought a gold sweatshirt and sewed a black collar onto it just so I could be Kirk for halloween. My insignia and other props were made from pizza boxes. It was decent for what it was. I have one of those rubies redshirts that I got at a costume store. I improved on it a little by having a better insignia sewn over the screen printed one. It's a little big, but some tailoring could improve on it. I wore it to the only convention I was ever at, but I half assed it and wore it with jeans hopefully telling everyone that I wasn't taking it very seriously.
But I've seen some of the great work some have put into their costumes and I am amazed at the detail, time and effort that went into them.