https://apply.ycombinator.com/
Less obvious, so worth adding: if you don't get accepted, keep applying for future batches. Repeat applications are more likely to get funded. The main thing is to demonstrate progress since the last time you applied.
Edit: also, YC is very invested in not overlooking applications from compelling founders who don't have any elite or industry connections. It's one place where you can prove yourself based on how good you are, not who you know. Not that YC is perfect in that respect, but it's something people here really care about and have a lot of practice with.
Edit 2: these recent comments from pg are also relevant: