Hive Tyrants are getting a new release this month, I think.
You'll be able to get them soon enough.
If you're looking for a good way to start off, it's best to figure out what sort of Tyranid army you're considering making. Do you want to focus more on shooting, or melee?
A lot of people seem to love the Tervigon / Termigaunt spam lists, which involve Tervigons focusing on creating TONS of Termigaunts every turn, and just overrunning the enemy with bodies.
If you're looking for melee, learn to love Genestealers, Trygons, and in some people's opinion, Carnifexes (in a squad).
I tend to like an odd combination for my Tyranids, primarily shooting, with some nasty melee elements. Hive Tyrant with long range cannon, two Zoanthropes (one of my favorite models), two Hive Guard (insanely useful), 2 squads of Genestealers (absolutely brutal when you can get them into melee), and Raveners (most people hate them, I like them a lot).
I'd recommend first and foremost buying the codex, and figuring out what you want to do with them from there. After that, it's mostly about figuring out what to buy and how to do it.
A word to the wise though, the Battleforce for Tyranids is... quite lackluster. You get no HQ unit, Warriors are mostly avoided by everyone, and really nothing overly impressive. You're almost better off building your army piece by piece (spoken from someone who bought it to start
).