What model of Dell notebook do you have? Some of them have a separate door on the bottom of the notebook which exposes the RAM insertion slot. If not on the bottom, the you will need to remove the keyboard. This varies some with different models, but basically you need to look around the perimeter of the keyboard for a small tab-in-slot fastener. There will usually be 3 of them and the most common location is across the top of the keyboard. You will need to VERY CAREFULLY pry the keyboard (with a small screwdriver) loose at these points. This is done by flexing the keyboard (bowing the middle portion of the board slightly up) until one edge of the board comes up. then slowly lift the keyboard up, hinging around the edge opposite the one you pried the board loose at. Be careful lifting the board out as you will want the cables at the lower portion of the board to stay attached. If you do have to unplug any cables, keep in mind that cables in notebooks do not unplug like desktops; the plastic connector on the notebook cables will pull slightly up, then the end of the ribbon cable will slip free.
Basic rule: don't force anything! If you're not comfortable doing this or if breaking something would be a total disaster, find a trained technician and have it done.
Good luck!