Stephen if you haven't played at espn before you might need to create a user there. however the invite should have a link... from that screen it will walk you through creating a user and then on to joining a league.
Once in the league look around and set your team name and such.
Big thing is you need to look at the draft time... That time (note it is Eastern, not Central time like all smart people use) is when the online draft starts. Basically you need a java enabled browser to hit the draft... I recommend going to the page 30 min early to make sure it all works.
Then you can draft your team. pretty standard scoring and setup for rosters... Might I recommend that awesome QB from the Vikings... Jackson is his name I think