ODST is set around the time of Halo 2 / Halo 3 IIRC, following on from events in Halo 2.
I guess without knowing the other games you will still be able to get the basic gist of it, but it would obviously help if you had played the others.
You could also just read up via the 'net...
http://en.wikipedia.org/wiki/Halo_%28series%29#Story
http://halostory.bungie.org/halostory.timeline.html
etc.
Regarding ODST though: I wouldn't bother with it at full price. I did really enjoy it (& the voicecast is awesome), but the single player campaign is rather short.