Find a music store that sells amplifiers and play through the amplifiers available to hear the different sounds of each one. Try to use a guitar similar to yours.
Decide whether you want a combo amp with the amplifier and speaker in one package or a separate head and cabinet. Combo amps are generally used for playing smaller venues.
Decide whether you want a tube amp, which utilizes vacuum tube technology and creates a warmer sound, or a solid state amp, which tends to create a cleaner electrical signal.
Decide whether you want an inexpensive beginner's amp or a more expensive professional amp that can be used to play most any venue.
Look at the quality of construction: thicker wood is usually better for a good amp sound.
Check out the power rating and speaker size. Higher powered amps are usually used in larger venues.
Plug your guitar into the amp and turn it on. If you have effects pedals, plug the guitar into those and plug those into the amp.
Adjust tone controls and volume controls before trying different effects pedals.
Turn up the treble to get more high end, especially from guitars like the Les Paul, which has a warmer sound than a Fender Stratocaster.
Set the mid range to a lower setting, around three or four, to get a good sound for rock.
Set the bass at around six or seven to get a good low-end sound, or set it higher if your amp sounds thin.