You've received some great advice, Jon, and it's nice to see members stepping up to help. When you figure which costume to go for, we're all here to help.
All the green marked square entries include pictures and a ton of detail on what is needed. I'd suggest browsing that and picking which one you'd like to go with. Decide if you want to aim for Level 3 right away, or level 2 or if you're just looking to get into the 501st with level 1 requirements.
after you've chosen what you want, all you have to do is start assembling your uniform. It's completely up to you what you want to start with. If you find anything locally or online, take some pictures, share them on the forum and if you're unsure, a lot of informed members on the forum can help tell you why or why not something is 501st approvable (or CRL level) based on images you share.
A great place to start on the forums I think is the "Getting Started" section, which has a bunch of information on reputable vendors for items. Tutorials on how to build your own uniform items and tips and tricks also exist on the forum.
http://www.imperialofficer.com/forum/index.php?/forum/57-getting-started/
http://www.imperialofficer.com/forum/index.php?/forum/94-tutorials/
http://www.imperialofficer.com/forum/index.php?/forum/35-howtos-tips-and-tricks/
Another place to start is the Introduction forum. Once you find which uniform you plan on having (or before you do that, it's up to you really), you can introduce yourself to the forum community, say hello! Share some information on who you are and what you love.
