My question is what level can you take a feat that has a pre-req of 'x' ranks in "skill"?
Example being mounted combat, it needs 1 rank in Ride. Can I take this as my Lv1 feat if I make sure to have put rank in ride Lv1 too. Or would I need to wait till 3rd level to take the feat?
You can take a feat that you 'just' qualified for at that level. Effectively you take feats as the last step of the leveling process, and you take them in whatever order you like.
For example, as a 1st level fighter, you could take Mounted Combat (if you also put 1 rank in Ride)
You could take any feat with a prerequisite of BAB+1
Also, say you are a fighter. You get two feats at first level. You can take feats in the order you choose, so you could take Power Attack and Cleave (prerequisite Power Attack) both at level 1.