Your sponsor will be the person that introduced you to the game. As you sign-up for Urban-Rivals a slot where you put your sponsor's details (Eg. player's ID ?) should be filled up. That is how you get a sponsor.
If you want to be a sponsor to someone then just simply invite them in the game and ask them to write you as their sponsor.
From what I've gathered, it seems like most people, myself included, have a spammer as their sponsor. Pkfist anyone? You type Urban rivals sponsor into google, you get about a hundred of this guys posts promising free credits, or free cards for using his code, which is all BS of course... Yet I see 20-30+ people a day joining under his sponsor code.. he must have thousands of "sponsorees" by now and he doesnt even seem to play judging from the fact that he's been level 6 for months... Really wish you could "unsponsor" someone...
But to answer your question, you can only get one when you sign up and the only benefit is to the sponsor.. theres no actual benefit for just having a sponsor. Its essentially just one-way referral bonus, they get like 10% of the credits you purchase for free.