This can be a bit tricky depending on your experiences and your organization. I have worked on projects which have taken a very mathematical approach as well as using a basic low/medium/high estimate on individual tasks.

There is an article from awhile ago here that talks about a more simple method which you should take a look at that can help you.

http://articles.techrepublic.com.com/5100-10878_11-6160572.html

Good Luck.