for 7k, you should experiment with different key bindings until you find what is most comfortable for you
personally I use a s d [spacebar] k l ;
there's a wide selection of ranked maps that are made for beginners
when i first started, i remember playing this map a lot just to get the muscle memory down https://osu.ppy.sh/beatmapsets/314954#mania/702436 these are some other 7k maps with easy difficulties that i used to play:
i'm not sure if this helps but these are key bindings I tried that didn't work for me:
i've used q w e [spacebar] o p [ before but I found it made my hands tired and made my thumb sore
i've used a s d [left arrow] num4 num5 num6 but the arrow key on my keyboard has an uncomfortable edge and it messed with my thumb
i've used a s d [spacebar] l ; ' but with this positioning my thumb wasn't hitting the spacebar as well as it could be
i hope this helped, 7k takes a little longer than 4k for the muscle memory to set in, at least it did for me
I currently use sdf op[. I used to use sdf kl; because jkl had ghosting issues.
Like SednA said, there is no right key setting. Find what works best for you. The thing is: you shouldnt even be worrying about your key choices yet. Key choice is a minor thing until you get much better.
The switch to the upper row on my right hand made a difference for me, but that was because I identified a problem (my left hand had better stamina) and I tested a change based on looking at how I physically held my hands over the kb and realized my right hand was more curled than my left because I had to hold my right hand higher on the kb to hit the space bar. I decided to increase the distance between my fingers and the spacebar by moving to the top row which let my fingers hit the keys on a lower angle rather than being curled downwards. This made a small but noticeable difference in my right hand's stamina. But I only made this change after playing mania for years with both hands on the same row.