Hello, I am newbie, and I have a question, what traffic source You recommend to choose and what nich? I am thinking about Facebook mobile traffic with games niches? What's your opinion? Thanks for help.
You've just asked a really general question. 
Here are my suggestions:
1)Read some of the recent threads in the "Success Stories" section - to see what kinds of stuff people are having success with.
2)Read some of the recent follow-alongs in different sections - for the same reason above.
3)Ask affiliate managers for advice.
Then make a decision.
Be aware that you don't need to be stuck with a vertical and traffic source forever! Just pick a vertical and traffic source that you KNOW work well together (i.e. lots of people running the vertical on that traffic source), and a vertical that has low payout offers so you can gather data faster and learn faster. Without data you can't learn to optimize your campaign. Your focus at this stage isn't to jump into the most profitable vertical and traffic type, it is to pick something that a lot of people are having success with, so that you can eliminate some uncertainties. Once you have experience you can test anything you wish - by then you'll know what kind of offers to run on which traffic type, and how to gauge whether an offer has potential, etc.
When you have your options narrowed down further, feel free to update this thread to get additional advice. 
Amy
In addition to the good advise (as always) from Amy, I want to add that although mobile games work very well with Facebook traffic, and Facebook in general is a great traffic source, its also not the easiest to start with. I recommend you really understand how FB works before starting campaigns there as it can burn money very (very) quickly and also its very easy to get banned.
I am not writing this to discourage you, just to try and emphasis that you need to understand the pros and cons of this (great) source.