I was seeing a friend playng Halo ODST and while it got my interest, I couldn't stop thinking how lost I'd be if I started playing Halo now.

The trick there is... Halo I and II were released to the original Xbox, which I never had. Halo 3 is for the 360 but I know II and III are kinda interwined. Halo ODST afaik is better enjoyed if you know something, anything about the franchise.

Long story short, the Halo series looks like a long, nice movie which I lost the start so I don't bother start watching now.