1. Should I only submit my article to one article directory? I've originally submitted to ezinearticles.com but I've later added them to 4-5 other sites to get more exposure. Is this a bad thing? I've heard it's bad for search engine rankings and other things.
I would submit all the places I can
2. Same as above, but what about squidoo, digg, and sites like that?
Squidoo fine, Digg youll have to be lucky or smart to see any results. Its also crappy traffic often (I once hit the front page for a few minutes / hours and the resulting 17,000 visitors didnt pay like they should

)
3. Should I only submit articles that I'm passionate about? I'm very much into health, fitness, working out and stuff along those lines. Should I even consider writing about something else?
The honest answer is write what you find easiest to write about as being able to write lots will help you in the long run, get used to spouting content out
4. What's the deal with keywords? This is what I've been doing. I'll go here:
/
I'll Type in something broad on the topic like "build muscle" and just add keywords in there from highest amount of searches down and keep going until I run out of room on ezinearticles
Bit of a black art, when you do a search for "build muscle fast" i see 453,000 competing pages in google. Imagine how you can compete to be in the top 10 most relevant pages for a term like that.
5. I've heard another thing about keywords. Say I make one of my keywords "build muscle fast". I've heard something like if I add that term in my actual article as much as possible to get higher in searches for that keyword. Is this true?
Not really, mention it sure, make sure its in your headline, title and the body somewhere a few times but more importantly make sure the links you get overall use the anchor (clickable text) of 'build muscle fast'
6. How can I get higher in search engines, or get more free traffic to my articles?
See above + links to your site using that phrase + more links + more links + some more
7. Any other common mistakes newbies make? Any tips for me?
Dont assume this is easy money, sell your traffic to aff first and adsense for the rest
8. I can do this if I'm 13, right? This is where I originally got into clickbank and article marketing:
Im 34 so no idea
/watch?v=5h_eelybJGk
He says you can do it if your 13, I just want some confirmation from someone other than him.
Dont believe what anyone says, experiment and learn
9. If I can if I'm 13, I don't need a PayPal or anything like that(that requires to be 18). I just request a check from clickbank and I'm set?
Suppose so, never tried. Ask them