Finger and hand position
Every guitarist starts the humble way.
Like riding a bike or learning to write, it's all about getting accustomed to the new movements.
But it sure is more fun to learn when you have an idea of how it's done.
Follow these tips and you'll avoid a lot of confusing moments and frustration:
To fret without pressing other strings you need to adjust your fretting hand position so that your fingers are almost parallel to the fretboard. That usually means you have to move your thumb more to the center of the back of the neck and turn your wrist to a steeper angle.
That might feel uncomfortable at first, so remember to give your hand a rest if it starts hurting. The optimal place to press the string down is right before the fret, not on top of it.
Check this quick video on how to do this: Finger and hand position video