This is a complex question. The answer will depend on a number of factors. The following list of questions will help you figure out what you need.

What will you be using the amp for? If you will just be using the amp for practicing at home, you will not need tons of power. A simple, inexpensive practice amp with 10-30 Watts of power will probably suffice. If you will be using the amp for live shows, you will need an amp with a little more power. At least 50 Watts will be necessary (although 100 Watts would be better).

What type of music are you going to play? Different amps will sound better for different types of music. For example, Marshall amps are great for classic rock and metal, while Fenders are great for pop, blues and country. Other amps may be more appropriate for your style.

Do you want a tube amp or a solid-state amp? Tube amps are typically more expensive and require more attention to keep in good working order, but they are generally regarded as having a more pleasing tone. Solid-state amps are less expensive and do not require any maintenance. They also often have built-in effects, like distortion, reverb, etc. However, solid-state amps generally have a harsher tone than tube amps.

How much can you pay for the amp? Generally, you should get the best amp you can afford. A great guitar will not sound so great through an inferior amp. On the other hand, you may find that a nice amp will make your inexpensive guitar sound better. Try to find a good balance between the two.

This should help you narrow down your selection. If you have friends who play the guitar, ask them about their amps. Listen to them play to see if the amp sounds good to you. If you are still in doubt about what to get, visit a local store and try out a few amps. You'll be sure to find something that works for you.