This is the academic's approach to learning a new system. It is quite heavily theoretical and assumes that you're willing to take time and research sources. If you just want to jump right in, take the PHB and make a character by hand, then use the various resources on the net to check your math.
For websites to help you narrow down choices for character creation, I recommend the various handbook compliations out there. While these are not written for newbies, these are effectively the distilled wisdom of many years of play. By using the core books, yourself and by hand (it's the only way to learn character creation, I've found) the handbooks to narrow the design space for the specific class you've chosen, and asking questions here about the validity and utility of specific builds, these resources can help you build your system knowledge.
There are some things to stay away from as a newbie. While caster classes are excellent, the sheer scope of spells that you need to consider between produces analysis paralysis among the best of us. Avoid Tier 1 unless you have a very specific and compelling need to play one of them.
Identify the classes being played and try to choose a class that's in the average tier. That way, you will not be too far above or below the party "capability" leve, allowing you to express your agency without obvious inequalities between party members. The first character I had was a bland fighter in a party of casters... and it just wasn't much fun. His job was to stand around and occasionally intercept someone going for the casters.
Once you've done that, and built your character by hand, the most critical element of system-mastery is the checklist. Identify, before the game starts, your common strategies, and the mechanical activities needed for those strategies. This "study" is important to build your own, internal, mental map of possible actions and pre-anticipation of situations in which to use those actions. It will also help identify if the design you've built meets your requirements.
Always do your initial builds by hand until you understand the math behind the scenes. I made the mistake of using tools for my first 3.5 characters and that actually inhibited my system-mastery for a year or so.