1) If you want to write your own fluff for a chapter, you are generally making a "Codex Chapter" which grants you access to the entire Marine codex. You -can- field special characters from other chapters according to the rules. Just cross the name off of the original character. Simply paint to taste, as has been mentioned before. Blood Angels and Blood Ravens have nothing to do with each other.
2) You're entering the game at a point when two specialized marine chapters have received insanely powerful new codices and the Sphess Mehrine Codex is nearing the end of its current edition run. Space Marines tend to get updated first, and the Marines were the first major army updated to 5th edition. Right now, the Blood Angels and Grey Knights codices are both very, very powerful while the Space Marines are middle-of-the-pack. The Space Wolves have also been recently updated and are much less OP. However, this is a cyclical thing; every army eventually gets its day in the sun, even if it takes more than a decade (poor Dark Eldar...they finally have a "good, not great" codex in fifth edition; their last update was 2nd edition). Before you start painting, pick up a how to paint minis guide; Citadel makes theirs, and it's good, but there are others. Watch a few tutorials, read some pages, etc. A lot of new people just dallop paint on and it comes out awful.
3) You didn't ask this, but a lot of new players -immediately- latch onto Space Marines as the army to be, only to find Marines don't really suit their play style or are less interesting to paint and model than something less "vanilla." I always recommend you find a group with a lot of different armies and see how everything plays. It spared me the terrible, terrible fate of being a marine player. Marines suffer a bit from being the "Mario" of 40k; they're good at everything but great at nothing and people expect to face them frequently in the metagame. This isn't to say, "Don't play marines!" but rather, "Try before you buy." I had two friends drop several hundred on a Marine and IG army, respectively, only for both to find the army they liked in universe wasn't fun for them to play on table top; the Marine player now plays 'Nids, and the IG player Orks.
Edit: typo fix