Lens for wildlife photography. Which one to buy?

I own a nikon d5100 dslr. Along with it i have a 18-55mm lens and a 35mm prime lens. Which other lens should i buy for wildlife phtography? And remember i don't have a high budget…

Wildlife comes in many shapes and sizes!
For insects then a 100mm macro lens
For "don't get too close or they run away" then a telephoto zoom up to around 250 or 300mm
For "if I get too close then I'm lunch" then a telephoto or telephoto zoom up to 500mm

Budgets also come in different shapes and sizes so I don't know what "high" means.
A decent macro = £350
A decent medium telephoto zoom = £250
A decent long telephoto zoom = £500-800
A decent long telephoto prime = £2,500

There are several tele-lenses that you can buy; however, I don't know what you mean by not having a "high budget." So, I would recommend that you look at the following URL and try getting a good USED tele-lenes at a reputable place like B&H or Keh.com. Here's the URL I ssuggest you check out:
http://photographylife.com/best-nikon-lenses-for-wildlife-photography

What sort of wildlife? How far away?

Unless the wildlife is big or close you'll need a 300mm lens at least. These were taken at a zoo with a 70-300L at 300mm on an EOS6D.

Buy a used 400mm or a zoom with that focal length and you'll be good to go. Lenses for wildlife start at 300mm, and even at that focal length it's considered weak for most shots.