Which lens should I begin with?

Well, I was originally going to buy the Nikon D5000 kit which comes with a 18-55mm.
But now I'm wondering if I should just buy the Nikon D5000 body with a 50-200mm lens, since it's nearly the same price.
Which lens is more idealistic for a DSLR beginner?
On a tight budget, so I will pretty much have to stick to one lens for probably about 6 months…

Added (1). Oh, and if this helps, I love taking nature pictures (animals, flowers, insects).

Depends what you are shooting, but I think the 18 - 55mm would be best. The 50-200mm will not be any use indoors as you are stuck at a 50mm zoom.
I have lots of lenses but I would choose the 18-55mm if I could only have one.
Use it for a while and if you need more zoom go ahead and get the 50 - 200mm. Those 2 lenses compliment each other well.

I have an 18-55mm lens and a 55-250mm lens.
If by "animals" you mean pets then go for the 18-55mm

If by "animals" you mean wildlife then the 50-200 will be best for that but not so good for the rest.

If you are on a tight budget why not drop a spec on the body and get a Nikon D3100 and both lenses.

The 55-200mm is a great lens, but you'll miss a lot of the wide angle, which will be used for everyday stuff, groups etc. I think it's best to buy with the 18-55mm.

All DSLRs will be great for what you said and all other stuff.

Here's a DSLR Buying Guide - http://www.the-dslr-photographer.com/2009/11/which-dslr-to-buy/

As you've probably noticed, an 18-55mm zoom lens is almost a standard with entry-level DSLR cameras. The reason for this is that camera manufacturers have determined that this focal length range will be suitable for about 95% of the pictures a beginner will take.

So in my opinion your best choice would be the 18-55mm zoom. While the 50-200mm zoom would be fine if all your photography was going to be done outdoors you'll find it all but useless inside your home. A 50mm focal length on your D5000 is equivalent to a 75mm lens on a 35mm film camera or full-frame DSLR which makes it a good choice for portraits but a very poor choice for group pictures.
Indoors you'll need the 18mm end of the 18-55mm zoom for group pictures unless your living room or family room is absolutely huge.

This site will illustrate what I'm talking about:
http://mansurovs.com/lens-zoom-on-dx-15-crop-factor-cameras

As you can easily see, 18mm allows you to include a large area as compared to 50mm. Now can you imagine trying to use a 50mm focal length indoors to photograph a group of family and friends?

So buy the D5000 w/18-55mm zoom and after 5 or 6 months you'll better understand the capabilities and limitations of the lens and can then make an intelligent decision as to what your next lens should be.