do R' U untill the cube is solved
If you use left hand for OH, these are the fingertricks I use:
R: ring finger/pinky toward you
R': opposite of R or index finger down toward you.
U: get a corner on FUL and flick towards the back
U': regular U' that you usually use on 2 hand solves
F/F': I ususally regrip and grip the entire F face then do basically a b or b' or rotate and do L or R
Get your finger tricks right, maintain look ahead and practise a lot.
I suggest buying a smaller cube, such as a mini Type A or A5(slightly larger than mini type A). That worked well for me when I was about your age.
Well, you've really got to learn to practice.
It really didn't take too much. After about a week or so of hardcore practice, I jumped from around 45-50 seconds to mid 30s, because I was able to learn finger tricks on my own, and learned where rotating the cube was faster than trying to do an awkward alg.
Also, you're not that fast at normal 3x3. Don't expect to be able to grasp OH easily if you're not even sub-40.
My advice: Get sub-30. Preferably sub-25. Know full PLL and quite a bit of OLL, and be able to easily and efficiently do all F2L cases. Learn how to influence LL edges to have easier OLLs. After you've gotten a grasp of regular 3x3, then bring what you know about that to OH. It takes practice to do well OH. Your hand will have to adjust to your style, and you really need to learn to adjust. If you think that you can get better at OH without practice, you're dead wrong. I know people who are sub-14 on 3x3, but barely sub-40 OH.
Overall, OH is probably the hardest thing to start, but exponentially gets easier. Practice, practice, practice.