Voter registration?
Not particularly well versed on this in the USA but it does depend on which state you live in.
Always remember, the USA is a union of states not a federation.
What this means in practice is that the states have a lot of say over a lot of things - including how voters register and how the vote takes place.
So, in some states it would be relatively easy to register to vote. In other states they make it much harder and ask for ID when you vote. Also some states go out of their way to remove voters from the electoral roll, generally they will do this more vigorously for poor and black voters. You also get states who will, for example, have 10 polling booths for 10,000 people in one area, and 1 polling booth for 10,000 voters in another area. If you want to stand for President you are not guaranteed to be on the ballot paper in every state, it depends on the state laws about this. There have been many elections where candidates have been on the ballot in some states and not others. The way you vote is different in different states - in some states (such as Florida - remember the hanging chads, that was only in one county) the ballot is different in different counties. Some states/counties might have voting machines (pull a lever, push out a chad etc) and others have a paper ballot which you fill out.
The President is also indirectly elected, see last night's Planet America for a reasonable explanation of this.
It is often asked why the USA has no Federal Electoral Commission like most countries. The answer is that a Federal Electoral Commission makes no sense in the US model. The states decide how voting, registration, nominating, appearing on the ballot etc happen. You can't have the same rules for the whole country as the election is far more state based.
So, voter registration? Can't give one answer because it is such a convoluted and messy system which differs from state to state. In fact, until the voting rights act in the 1960s many Southern states would use various nefarious means to prevent blacks from registering, and that act has been undermined over the years.
Sorry I can't be clearer, I really don't know the details but the above is what I do know.
DS