I am trying to convert my garage, and the garage door wall has a header on it. I am trying to frame a wall with a rough opening for a window and was wondering how I would fit the king studs, do I build a new header underneath or just have two jack studs?
It depends on the window size to some extent, but typically I'd not add another header. In fact, some carpenters put all headers against the top plate, with pin studs down to the RO.
Just drop your studs in and build down (if necessary) by whatever means is convenient. You always want at least two members along a door or window for rigidity, curtain backing, etc. Tie it all together well with sheathing and be happy.
Installing a new header or modifying the existing header without accommodating the load transfer to the existing footings will CREATE problems.
I’d add a beam on the sole plate (probably the size of the existing beam) to transfer the load from the header over your new window to your existing footings.
In addition, I’d provide a sealed drip edge at the existing concrete. By code, lumber within 6” of the ground (your driveway) needs to be “treated”.