Whichever distribution you choose (and Gentoo is most definitely not for a first-time user) go to their website and read the installation documentation. There is little point in attempting to reproduce in a forum post what is already comprehenfsively documented.
Before going further you might like to think how you use your computer, what it is that you dislike about Windows, and why you think Linux would be more suitable. For example, if you play games then you may be disappointed with Linux; on the other hand, if you are a programmer then you will love it.