If you're looking to buy a guitar amp, there are a few things to consider before buying.
Where are you going to be playing?
If you're a beginner guitarist and plan to play mainly without a band in a small to medium sized area, a good combo amp is perfect. These amps will give you a good tone at lower volumes for practicing. Choose anything from solid state practice amps, small to medium solid state combo amps, or smaller, low wattage tube combo amps. There are many small amps perfect for playing at low to medium volumes.
If you're planning to, or are playing with other musicians in a band (especially a drummer) you'll need a bigger and louder guitar amp to be heard. Larger combo amps, half stacks, and full stacks are ideal for playing at medium to high volumes. Tube amps tend to play their best at higher volumes, and are a popular choice for guitarists planning to play shows with a band.
What style music are you going to play?
Amps that produce a higher amount of gain and distortion are typically the choice of rock and heavy metal guitarists. With all the distortion and overdrive effects pedals available you can add more distortion to a clean sounding amp. Amps with a cleaner, less distorted tone are ideal for blues, jazz and country guitarists. Open back combo amps played slightly overdriven are a good choice for blues guitar tones. Half and full stack amps are louder and typically more distorted in tone which make them a good choice for hard rock and metal guitarists.
What is your budget for buying a guitar amp?
Just as amps vary in power and size, you'll notice the price tags of guitar amps vary just as much. If you are considering a tube amp, you are likely to pay more to purchase as well as maintain your tube amp. Tube amp maintenance includes costs for new output and preamp tubes, and other related costs. Though there are solid state amps that may be more expensive to buy, solid state amps are a lot easier and less expensive to buy and maintain over the time you own the amp.
Buying a Used Guitar Amp.
Sometimes you can find a good used guitar amp at a lower price than if the amp was new. Be sure you can try out the used amp before you buy it. Be sure the amp works well and doesn't make an suspicious, unwanted noise.