As a battle-forged army?
Patrol ... 1
HQ and a troops choice. The "Start Collecting" box has you fixed with a Hive Tyrant (
HQ), Warriors (Troops) and gargoyles (Fast Attack).
People tend to play in 500 pt chunks, so, aim for a 500 pt army, then start building towards 1000 pts, then 1500, and so on.
Never be afraid to try out new units, but keep the spending to a reasonable level. A good rule of thumb is a box a week allows you time to buy, build, and paint, but that can hit $200 a month, which is out of line for many people. A box a month is easier, but can be frustrating because of how slow the army grows.
You'll have to find what works for you, but, Start Collecting is the best way to start and, in all honesty, a second one's not bad either.

After that? It gets trickier.
Automatically Appended Next Post: Like, links. You can usually find better deals at your friendly local gaming store, but, I'll plug the official
GW site for the moment because it's easier to link and shows things off well.
https://www.games-workshop.com/en-US/Tyranids-Starter-Collection-ENG-2017
That's the whole kit n caboodle, with the Start Collecting box and the Tyranid Index with all the rules for your models. A listing ithout the Index also exists, for cheaper, in case you want a second one.
MOST people will tell you to add Genestealers ASAP.
https://www.games-workshop.com/en-US/Tyranid-Genestealer-Brood
I'm not in that group since I find them too fragile for the cost, but they absolutely hit hard, so if you're an aggressive player, they're great. These will help you add more Troops options, which you always need.
https://www.games-workshop.com/en-US/Tyranid-Termagant-Brood-12-Models
These are my preferred troops. Fragile also, but dirt cheap, and ranged attacks give you nice options. Small numbers hide well for taking objectives while large groups dish out firepower. If you go this way, you'll eventually want quite a few. The unit of choice these days is 20 with Devourers backed by 10 with Fleshborers, but I like Fleshborers better myself ... they don't hit as hard but are cheaper, so you get more wounds out there, which means you live longer. (I play differently than most) ... regardless, 2-3 boxes of these guys will really help flesh out your force.
Mess around with these basic tools for a while, then come back and we can help you with some more advanced ones. Many people get the first really big Tyranid model and decide that they want to go all Godzilla from then on. Others give in to the swarm and focus on piling up massive numbers of squishies. Your mileage may vary.