I Want to Collect 'Nids, how do I Start?

You should look at what type of army you want to field; for example, swarm, 'stealer shock, Nidzilla, shooty, or assault.
This will tell you how much synapse you are going to need. If you rely primarily on 'stealers for your troop choices, you won't need much. If you want 300 gaunts, you will need a lot more.
Warriors are a bit tricky. They are not cheap points wise and this gets to be problematic when you start stacking biomorphs. By relying on them for synapse, you will need to field a LOT of them since they are fragile and a canny player will target your synapse creatures first. This is the main reason I rely on Tyrants (one walking with long ranged weapons, +EC, +TS, and one flying with 2 sets of twin linked devourers, +TS, +warp field). If not two, then I will use my Flyrant with a Broodlord (infiltrate as a fire magnet, plus synapse in the front of the battle line). Either way, I like to balance out synapse cover with Zoanthropes.
Remember to check out Zoanthropes - not only can they provide synapse with superior armor saves, they have str 10 warp blast.
Tyranids are one of the most customizable army in the game (if not THE most) so answering these points will give you a better idea of the direction you want to go without buying a lot of minis that don't suit your play style.

I agree. My recommendation to start off nids (I play them, but you should be willing to pay quite a bit of money):

If you cant spend tooooo much (depends on your opinion of too much, as too much does not exist in warhammer ):
2 Battleforces
1 Tyrant
1 Gaunt box
1/2 Zoanthropes
That is a VERY good starter army, you get: 6 warriors, 2 carnifexes, 1/2 zoan'z, 24 Termegaunts, 24 Hormegaunts, 16 Genestealers, and a hive Tyrant. Great "core"

If you want to get fancy:
2 Battleforces
1 Tyrant
3 Gaunt box
3 Zoanthropes
1 Broodlord
2 Lictors
4 Raveners
This list give you more options and diversity, eg. you can make multiple lists, depending on your enemies style of play/army, and its really a blast to have so many models, eventually if you get a bio-titan off of forge world or scratch build your own, then you got a lovely Apocalypse Army! But lets see here, you are spending around what, $627.15 Canadian, eh?
Now that does not add too much, It's along the same lines as the one above, But if you really cant buy too much,
2 Battleforces
1 Broodlord
1 Gaunt box
Lets touch up on this one:12 stealers and broodlord, 24 gaunts, 24 hormegaunts, 2 carnifexes, and 6 warriors? really, you cant go wrong here. But remember to chose on what you want not what you think you should get.

Hi,

Tyranid are expensive to buy but rewarding to play. Depending on the army you wish to field, A Hive Tyrant is a must! They are just too good to pass up! The models are great and can act as a really nice centre piece on the board if painted and converted well. They are flexible enough to find a home in any list - an obvious choice in a monstrous creature orientated list, a key model in a wee gribblies list and useful mobile fire support in a genestealer shock list (depending on points).

Being an expensive army, it is important to plan out the army in advance with care. You need a focus to drive your unit selections. Maybe proxy units before you buy them as it can be disappointing to buy a unit which, on paper, is full of potential but fails to live up to expectation (third ed lictors and raveners being good examples).

A good tip for painting tyranid is to keep the paint scheme really simple. My original army had a terribly complicated paint scheme and so I never got round to painting most of them because it was so time consuming!

Hope that helps and all the best,

Zedrin