Anything I try to do requires reputation. How am I supposed to start using it? I cannot reply to answers. I cannot comment. I cannot upvote.
I thought the same as you when I joined..
"Huh? You wont let me make simple comments but I can answer?? Doesn't make sense"... but it's like that for a reason.
The answers and questions can be community moderated, that is downvoted for those being a nuisance or providing bad answers/spam/etc, and upvoted if it's on topic and good quality etc.
Then once you've asked or answered a bit and suggested some edits on poor questions, and earned some rep (aka trust) you'll get access to comments and other things.
This process also gets you accustomed to Stack's ways, which being a Q&A is quite different to most other sites like forums etc.
Just make sure to answer on things you understand well, and take a little time so it's good quality, you'll get upvotes in no time.
It doesn't take long. Really doesn't.
Stack Exchange sites require points (reputation) to do these things because reputation is a (very rough) measure of trust. Once you have a few upvotes on a question or answer, many of these privileges are unlocked.
To start participating, just ask or answer a few questions! If the community thinks that they are quality, they will upvote them. You gain reputation from this, unlocking more advanced privileges.
Also, you could suggest edits to improve questions and answers. Every suggested edit is reviewed by two (three on SO) people, and you get +2 for each edit approved.