Help with a telephoto lens?

I'm looking for a nice telephoto lens to start out with. Preferably not too pricey. I have the Nikon D3100. I don't really care what brand lens I just want some ideas. I have already looked at Nikons website, but I'm not really sure which one would be best for me, so I want some input. I would be using lens for taking pictures of sport activities. So any help would be nice. Every time I ask photography questions on here, I get rude answers, so friendly advice please!