First you take your rectangular prism and flatten it so you can see all the sides. This is called a net:
Now take the measurements for the height, width and length or each part of the net (top & bottom, left/right side, front & back) and find the area of each. Since the top-bottom, sides and front-back are the same, you only need to measure one part (top, left side and front) then multiply what you get by 2 to represent the bottom, right side and back.
Then you add everything together. There's your surface area. It should look something like this: To find the volume of a rectangular prism, all you need to do is take the length, width and height measurements then multiply them together. Here's an example: Well... I'm done.