Iv said it before and Ill say it again. Your army is your army. Do what you want with it and make it an expression of yourself.
You want to go generic space marine chapter? Or go for one of the codex chapters, like space wolves, dark angles, blood angels, or black templar? If you go generic you can come up with your own army, background, and scheme based on your play style.
Mine, as an example, are a Dark Angels sub chapter, who are in the service of a line of Inquisitors that discovered their dark secret, so I usually have Inquisitor allies, and alot of my units have served in the deathwatch.
Orks are even more fun to where you can make up your own Clan. Be it a speedfreek type clan based on alot of bikes and trukks, or, again on personal taste, a Mek army that employ alot of captured tech.
Read some fluff and get creative.
Should you need any more ideas or inspiration, try both the fluff section of Dakka, or sites like Wikipedia or
http://wh40k.lexicanum.com/wiki/Main_Page