you should already know how to solve the Rubik's 3 by 3 before learning to solve the 4 by 4 but if you don't that's ok it will just take a lot longer to grasp.
Step 1: Terminoligy
Step 2: Centers
There is basic just this one move to finish this step you can do the same move backwards if the pieces are in a different order. Like this r' u' r that's if they're switched. and if you have a piece on the back of the cube and you are trying to get it to the front just do this. l B' l
so just do that a bunch of times until you have every side together.
Step 3: Are your centers in the right places?
Get BLUE on the top. then look down on one of the sides you should see Yellow then looking to the left of you yellow should see RED more to the left is WHITE more to the left is ORANGE than on the bottom should be GREEN. I like to remember the order by saying blue mustard blood milk oranges lettuce.
If they are not in that order you will have to do this move to get them in the right place.
picture will show you how to switch them. odds are it wont only be one that needs to be switched some times you might need to move al of them but if you pay attention while you make the position of the centers that will help.
ok so you have some off brand cube and that's not the right order. well look at the corners they will show you the way.