HTML and CSS are the main criteria to get yout head round (You obviously have database knowledge? If not, good database design is essential
Once you get comfortable then start getting your head round TDD - Testing is essential in Rails, far more so than when developing desktop apps as rails is interpreted not compiled therefore you only find typos at runtime (No compiler to pick them up for you) but good ide's help here as well, unfortunately the best IDE (netbeans) has just dropped support for RoR. RadRails is alright but not brilliant, I've just started playing with RubyMine which seems to be a pretty reasonable replacement for netbeans (Paid for) option
The totally superb "How I test" railscast is a great intro into RSpec, factory girl and cucumber which IMO is the ultimate combination
Then you need to start getting your head round JQuery - This is waay simpler in Rails 3.1 to use than previous versions of rails
What you want and what you need are too often not the same thing!
When your head is hurting from trying to solve a problem, stop standing on it. When you are the right way up you will see the problem differently and you just might find the solution.
(Quote by me 15th July 2009)