As long as you are above the atmosphere, you can circularize your orbit with: towards (aim straight at the ground), away (180 degrees from towards), prograde or retrograde burns:
If you are at apoapsis, burn prograde until your eccentricity is 0.
If you are at periapsis, burn retrograde until your eccentricity is 0.
If you are past apoapsis, dropping to periapsis, burn away until your eccentricity is 0.
If you are past periapsis, rising to apoapsis, burn towards until your eccentricity is 0.
The key here is get your eccentricity to 0. Then you're in a circular orbit.
If you're confused, aim any direction and burn. If your eccentricity increases, fire retro rockets instead (or turn 180 degrees and burn). Keep doing this until you've got your ecc as low as possible.
This will give you a good feel for orbits, and eventually you'll learn to get more efficient. For example: from launch, keep burning until your apoapsis equals your desired circular orbit. Then kill thrust and coast to apoapsis. Turn prograde and burn until your eccentricity is 0.