I'm at a similar point myself, but probably not agonizing about it nearly as much. From what I hear around here, if you question, then you are probably trans in some sense. Cis people don't question their gender. And just because you didn't feel it at age 5 doesn't invalidate what you feel now. I didn't either.
That said, how far a person wants to go with it seems to have a wide variation. Some people never express it out of their own private place, while others plunge into transition in all its aspects. With everything in between. And likely your own feelings will change over time. So you can just do whatever helps you be comfortable for now.
I'd suggest trying dressing, and as suggested above, see how it feels. How to get clothes? It's easier than you think. I've shopped as a male at an used clothing store, Walmart, Penney's and other places. The only question I've gotten was from a Walmart clerk to make sure I wasn't confused about which jeans to get ("Did you want the mens?"). That said, it can seem very difficult to go in there, but just do it. Nobody will bite or scream. If you have a female friend who will go with you (or your Mom!) it will be easier.
As far as your sexual preference, I've always been totally confused about terms such as homosexual, lesbian, gay in conjunction with trans people. It's like having a double negative, and I think the meaning of it depends on knowing how the person feels about themselves -- am I male or female? As my own feelings about myself transition, I might go from "heterosexual" to "lesbian" while never changing my attraction to my cis female wife. People around here don't use that terminology much, probably for that reason.
Ok, enough of my rambling.