First, welcome to FlyerTalk!

Second, that's a lot of questions with answers that must be tailored to your particular situation. You should look at all the airlines available from your location and figure out if there's any particular one you want to fly on, considering flights and availability from your home airport.

But, it doesn't stop there. Since most airline miles are now earned through means other than flying, especially with credit card bonus offers, you also need to look at airline alliances. If you're in or near a top 50 metro area, it's very likely that each of the three major alliances (SkyTeam, StarAlliance, OneWorld) has a representative airline flying out of your airport. I could write pages on this topic and plenty of folks with better minds than mine have done exactly that. The one sentence explanation for the importance of alliances: frequently you'll find that an airline award using your earned miles through another airline in an alliance may provide you a better redemption rate than the airline with which you earned the miles.

Finally, be realistic about what you can do. If you're young with little or no credit history and the type of entry-level income you'd expect at that age, you may have to pick and choose your spots on credit card applications. Credit score may affect how many cards for which you can qualify; income may affect the kinds and numbers of qualifying spending limits you can take on. If you fit the younger, entry-level end of the spectrum, you may want to hold out for cards that have a low or maybe even first-purchase spending requirement; if you're more creditworthy with some discretionary income to run through a credit card, your options may multiply.

I'd suggest checking out these blogs for more information as well as becoming much more familiar with the topics covered here at FlyerTalk:

http://thepointsguy.com/
http://www.frugaltravelguy.com/
http://boardingarea.com/blogs/viewfromthewing/
http://boardingarea.com/blogs/onemileatatime/

There are many others, but these are a good start. There's some especially big news today on the card front so this isn't a bad time to become familiar with these resources.

Good luck!