Of course you can just use headphones. BUT.....it's always good to have monitors as well.
Personally, I've found that it's good to have multiple monitoring sources to make sure your
mixes translate well. I do use headphones but it takes time to get good at mixing with them.
AND if that's all you plan to use DON'T skimp get the phones you can afford. BTW...aside
from headphones and monitors, it's good to check your mix on your phone and/or computer
as that what most people will be listening to your music on.

That being said we can help you better now and in the future with a bit more info......if you will post your system specs in your signature, (see my sig), we will be better able to troubleshoot your problems. Without knowing what type of pc you're running, what OS you're using, what DAW etc....we often can't recommend a "simple fix"

If you have not already done so please READ THE MANUAL!