Start with the oil pump. I say that because you say you are a little new to this, and while changing bearings isn't the hardest thing in the world it does require a little knowledge. As for how to know what bearings you would need if you do go that route, each bearing will be marked on the backside, and say something like "std" or ".010" etc. That will tell you if you have standard sized journals or if they have been turned.

The condition of the old bearings will tell you a lot too, but once again, you might want to enlist the help of someone who has been down this road before to give you a hand.

Don