First, consider whether you really have a home to give to this cat, and are you prepared for the months it could take before he's any 'fun'? During that time he'll hide behind your furniture and look at you with suspicion.
If you're sure you can offer him a home, make certain that he is a stray. Get a paper collar and put your details on it, asking his/her owner to contact you. If you have no response, you'll need to borrow a feral trap from a local cat rescue.
When you have caught him/her, keep the cat in for at least two months, longer if necessary. During that time pusscat will hide and hiss at you. He's not being nasty, but defending himself. Give him peace and food and a litter tray. To begin with, you may have to put him in a unused room while he gets used to your feeding and litter changing visits. Don't attempt to stroke, fuss, or pick him up.
After several months, this cat will creep out from his hidey hole and start to investigate. Let him go at his own pace, don't lunge at him.
One day he'll come and sit himself on your lap. But this could take months. However, when you achieve this, it's a wonderful feeling.