As someone who played cricket for 30+ years as a left arm seam bowler at a not-very-high-level-at-all, the best advice I can possibly give to any young bowler just coming into the game is forget about having too many variations until you can consistently pitch the ball on or around the leg stump (your natural angle to the right hander will be 'pitch leg, hit off'.
I used to practice for hours on my own with half a dozen cricket balls and one stump in the nets in order to get used to varying my length - I used to place a handkerchief on the ground and concentrate on hitting it. Being able to effectively change your length gives you the ability to 'work' a batsman around the crease.
The only real variation I used apart from length and swapping the shine over (I swung the ball a lot when the conditions were in my favour) was a slower ball, and this I delivered with exactly the same action but with my first two fingers very wide apart on either side of the seam. Try it - it makes a dramatic difference to the delivery speed, especially if you don't 'snap' your wrist forward.
Nigel Clough's Black and White Army, beating Forest away with 10 men