How do i get a stable orbit?

So after hours of frustration i can get to orbit. I have my ship in 3 stages to get into orbit, boosters then two engines with about 4 tanks of fuel each. However i can never get a stable orbit.

I use the boosters until they run out then open the map and when i see the apoapsis marker hit 70km i turn 90 degrees, Then decouple the first engine and use the second one to get a orbit. I always end up with a apoapsis of about 120km and a periapsis of about 60-70km. I can never get a orbit where they are both the same.

Does anyone have any tips or know any good videos that explain how to get into a orbit well?