If you have ODST then play it.
The story is good, it's a very pretty game, and you don't need to worry too much about the lore/story, since it's more of a spin-off than a part of the main storyline.
It's got an interesting system in the game's storyline, where you have essentially a free-roam around a city, and then everynow and then you start a mission where you play as other characters in a sort of flash-back.
The story for each of the characters is interesting, and so is the overall storyline, and it even has a hidden storyline that you uncover as you go along (Look/Listen out for car alarms, telephone booth chimes and cash machines!).
The Multiplayer isn't the most fantastic aspect of the game however.
There's 2 player local co-op campaign, plus a 1-4 player online co-op campaign.
And then after that I think it's just a 4 player Survivor style gamemode.
That's fun too, and it has a bunch of different maps where you can players fight off waves of enemies, which can be customised to an extent.
But you won't get any PvP play, since that's really provided by Halo 3.
I thoroughly enjoyed playing Halo 3: ODST, and it's one of my favourite Halo games.
It is definitely a good place to ease yourself in to the series.
And if you enjoy it, then you just have to consider trying other games. For a next step, I would recommend either Halo Combat Evolved (Halo1), Halo 3 or Halo Reach.
Combat Evolved is an aged game, but still feels good (Why not look into the Anniversary Edition?). And it's obviously the best place to start for the series.
Halo 3 is the next most similar to ODST, so you'll know what you're getting into. But you might feel a little bit lost for plot (or at least, the plot so far).
Halo Reach is then an improvment on Halo 3 as a game, and it's storyline is actually a prequel to Combat Evolved, so you'll have a great game with brilliant multiplayer and you won't be missing out on storyline for the campaign.
