I am astounded at the amount of bad advice that is given to people asking about backlinks and the disavow tool.
I went to the BrightonSEO conference on Friday (The big one in the UK) and here is the message that was being put across time and time again:
Don't link build unless you are very careful to create only natural links that real people will actually follow. Google have already blacklisted 100's of high profile websites because of poor backlinking and the news in the underworld is they are going to crack down on directory farming and splogging next.
Using the Disavow tool can do you no harm, unless you disavow a popular link that people actually use. If the link doesn't generate any traffic, then the disavow tool will have no ill effects.
This is the most truthful piece of advice I've ever heard:
"Links don't get you traffic. Good content gets you traffic, which in turn gets you links"
I know that's not very helpful to someone just starting out, but it'll save you a lot of time in the long run just to concentrate on good unique content.