Buying an Amplifier
For practicing at home, 5 - 10 watts output is fine, to play with a band, you will need min. 25 watts RMS. Combo means it has a built in speaker. As a rule, valve amps sound best (most pros use valves), but solid state(transistor) is catching up. Some amps (eg Marshall VS series) are a mixture of valves and solid state.
A recent development is the 'modelling' amp, which uses a microprocessor to simulate valves - made by Line 6, Roland, Yamaha. However, the real test of an amp is how it sounds on a gig - many players (myself included) still swear by valves - the rest may sound OK in your bedroom, then duff live. Loads of extra effects do not necessarily a great sound make - two really good sounds, clean and dirty, are all you need.
Also, don't buy any bigger than you need - 100 watt amps aren't necessary anywhere - a smaller amp like an AC30 turned up (still very loud) makes the output valves work harder and sound better than a 100w stack on 2. In monster venues, they'll put your amp through the PA anyway.
Decent makes: Marshall, Laney, Fender, Peavey, Ashdown, Carvin - these all make both valve and solid state amps.