If you want to post a new idea from inside a forum, you can start typing into the search box and then you'll see a "Post a new idea" button, or you can just go directly to the "Post a new idea" link on the right without searching at all. Again, however, you probably want to search if you havent' already to see if there are any similar ideas that have already been posted. There's no point in splitting out the votes when the task is the same!
Once you're in a forum or you've clicked on an idea and have found one you want to vote on, you can click the "Vote" button inside the idea. You can see on the right hand side of the screen how many votes you have left, and once you click the vote button you'll be able to choose the number of votes to allocate to it. If you just want to make a comment, but not vote, you can do that by typing in the comment box and clicking "post comment".