Bait packets are the best way to get rid of rats. Use gloves when you handle the packets to avoid contaminating the packets with your scent. Place by rat holes.
Definitely call an exterminator. Rats can be dangerous because they jump, bite, and can carry diseases. Also, your house could be condemned if you don't get rid of the rats.
Start by cleaning everything out of your basement. Then properly clean all surfaces. Next buy rat traps or rat poison. Make sure you don't use this around pets or small children.
The best thing to do is to put some traps out and put peanut butter in each one and make sure it is stuck on there. Also you could try to put rat poison out also.
Poisons and traps are the best way to eliminate your unwanted rat population. The best thing though is to make sure your not creating a place FOR them. Get rid of sheltered or food ridden areas rats may be involved with.