First, the only thing rep is useful for is the fact that you need I think at least 10 to be able to post links. Past that, as Melvinica said, if you want rep to enter giveaways, asking how to gain rep is not the way to do it. Also, be aware that "reputable" doesn't mean having high rep, but actually being known for your positive contribution on the forums for a while, so looking for ways to be able to enter faster actually harms that "cause" if that's what you're looking for.

But in itself, the first post you post each day (in your time zone) grants +1 rep. Only the first post per day, so not +1 for each post.
Then you get +5 if you answer a question, as in posting something in a topic marked as a question by the one who started it that the one who posted the question chooses as the answer. There can be only one post chosen as the answer per topic, and again, this only applies to threads marked as questions.
Past that, users can vote on posts by clicking the + or - signs, and the forum calculates each post's score. For each post you have with a slightly positive score, you get +1. For each post you have with a highly positive score, as in showing up as "high rated", you get +3 (or +2 in addition to the +1 it already had before hitting the threshold, if you prefer).
This works the reverse as well, so for each post you have with a slightly negative score, you get -1. For each post you have with a highly negative score, as in showing up as "low rated", you get -3 (or -2 in addition to the -1 it already had before hitting the threshold, if you prefer).
Posts flagged as spam and removed for it may come with worse penalties, maybe -5, but I was never sure about that.
There's also a maximum total number of points your rep can change by, whether up or down, per day. I believe the number's 12, but I'm not sure of that either.