Your cable modem opperates on normal CATV frequencies so typically a normal CATV splitter will be fine, but there are a few watchouts.
1. Only use a 1-to-2 splitter that has bi-directional capability. For every split you drop the available power by half. Since you want a clear signal split the cable as it enters the house 1-to-2 and put the cable modem on one side and any TV's on the other. If you need to amplify the TV signals, put the amplifier on that leg. The "bi-directional" part is critical since you need to send signals back (upload). This is also important if you have a digital cable box where you need to send "requests" to the cable company :)
2. Frequency in the past has only needed to go to about 900MHz. The recent adoption of DOCSIS 3.0 though has extended the frequency range to 1.0GHz. See http://www.cable360.net/ct/strategy/emergingtech/34304.html for some details. Thus you'll want to get a splitter capable of at least 1.0GHz range.
3. Quality. While you might be able to get a splitter that works at the dollar store, don't always count on it. On the flipside, you don't need to spend a ton of money for a name brand. I've alsways had good luck with these. http://www.monoprice.com/products/product.asp?c_id=104&cp_id=10422&cs_id=1042206&p_id=2872&seq=1&format=2 The frequency range is more than you need, but the price and build quality are both good.
