Is it advisable to name my startup www.teamsomething.com if www.somethingapp.com already exists?

Firstly you need to check if "something" is a trademark. Check it here: http://tess2.uspto.gov/ If it is, don't use "something" in your domain or you'll have trouble.

Secondly, is "somethingapp.com" a famous site known in your client base? If yes, don't use it as branding will be harder.

Better to use something unique if you want to go big.

Evaluating a trademark infringement risk depends on a lot of details. Two important factors are:

(1) The distinctiveness of the word "something". See here for details. The question is whether the word is fanciful, arbitrary, suggestive, descriptive, or generic. If the word is fanciful then you are in trouble, and if the word is generic then you are not. If the word is one of the other three types, then whether there is trademark infringement depends heavily on the second factor below.

(2) How similar your business is to theirs. If the other company is selling toothbrushes and you are selling motor oil, then you are probably ok, but if you are selling toothpaste, then you are probably not ok.

Entirely apart from the legal question is whether the other company would care or would be likely to do anything. If the other company is small, they probably don't enforce their trademarks. If the other company is in another country then they are less likely to take action since trademarks are country specific.

If you do use the name, the likely worst case scenario is that you get a nasty letter from a lawyer and you then change your name.

Sometimes the product name is more important than the domain name.

Example: Basecamp, a 37signals product - couldn't get the domain basecamp.com. So they used basecamphq.com. Eventually, they acquired basecamp.com - but the product name remained.