Check your bill to see if you are being charged a lease fee for a modem. It will usually be around $7 a month.
Instead of leasing it from the cable company, you can buy one for around $80 and save the $7 a month fee. You break even in less than a year.
One of the best modems is the Motorola SB6121 SURFboard DOCSIS 3.0 Cable Modem which is very fast and reliable.
Check with your cable provider to determine the right class of modem you need. Comcast currently requires a DOCSIS 3.0 modem.