Today's operating systems are multitasking, so you can have many applications running at the same time. Some applications access a lot of memory. Nearly every computer can benefit from a RAM upgrade. If you put demands on a system that is low on RAM, it has to swap some data from memory out to your hard drive to accommodate new memory demands. This swapping, or paging as it is also called, is a very slow operation compared to reading from and writing to Random Access Memory (RAM) chips. Depending on the type of computer you have, adding RAM is usually something that a moderately experienced user can do rather easily. This article assumes you already know how to open up your desktop computer case.

1. Figure out how much RAM you need. This is not entirely simple. For a Windows machine (assuming Windows 2000 or XP), right click on the task bar at the bottom of your screen and select the Task Manager. Click on the Performance tab. Now, load up as many applications as you typically have open simultaneously. Then open a few more as a fudge factor. Look at the MEM Usage Window in the Task Manager. How many megabytes are you using? Look to the right at the Physical Memory (K) Total numbers and how much is Available. Do you have lots of RAM left over? You may not need RAM. But if your Available memory numbers are very low, then you definitely need more. For a Mac OSX memory usage check, visit the OSX FAQ (see link at right). For Linux, check out the link for the Linux Journal article.
2. After you find out how much RAM you already have, determine how many empty slots you have available for adding more RAM. Your owner's manual is the best bet for locating the RAM slots on your motherboard if you do not recognize them. If you have no manual, download a manual. For a desktop, find the slots with RAM sticking up vertically from the motherboard. Usually there will be two or three slots. If a slot already has RAM in it, then it obviously can hold no more. If all your RAM slots are full, you will need to remove some RAM to add more RAM. That is an unfortunate reality. How much RAM you add is a factor of how much you need balanced against what you can afford. If you are a casual user who normally runs only a few office applications simultaneously, 128 MB is probably fine for you, and 256 MB almost certainly is. If you are a power user, even a gigabyte may not be enough. Look at the Available numbers from your Task Manager test above. Discuss this with your techie friends and relatives. Make a smart choice.
3. Figure out what kind of RAM you need. This is critical. There are many types: SIMMs, DIMMS, DDR, different speeds, and other variable. If you add incompatible RAM, things are not going to work well, if at all. You REALLY need a manual, or email the technical support folks for your motherboard manufacturer. Get the fastest RAM your motherboard handles if you are replacing all the RAM. Get the same speed you already have if you are adding additional RAM. Take the time to do this step right. Google the existing part numbers. Take a stick of RAM out, and take it to your local computer store. Get the right RAM.
4. If available, buy from a large local computer retailer if the prices there are reasonable. Mail order would be even cheaper, but you will have to wait. And if something goes wrong, fixing the problem takes much longer by mail. Figure out what you are going to spend and where, make your purchase, then add your new RAM.
5. RAM is very sensitive to static electricity. Your RAM will be in a metallic plastic pouch or in a plastic case with some black conductive foam. Ground yourself before touching RAM by touching a water faucet. I like to work on the kitchen island when handling static-sensitive parts. I reach back, touch the faucet, and then I work. I haven't zapped anything yet. Also, never work inside a computer without unplugging the power cord.
6. At each end of the RAM slot you will find a plastic lever. Press these levers down to eject RAM from the slot or to get the slot ready to receive new RAM. The RAM sticks are keyed so that they can only be inserted one way. Match the keys in the slot on the board with the notches in your RAM. You will not accidentally insert the RAM backward. Press down on the RAM stick carefully, but firmly. If you meet significant resistance and the little levers at the ends do not start to move up, turn the stick around and try again. Be careful that you do not push down on anything except the RAM stick. If the RAM still will not click into place, stop. Something is wrong. Someone may have sold you the wrong kind of RAM. Call technical support wherever you bought the RAM, and describe the problem.
7. Adding laptop memory is either easier or so much harder that you may not want to even attempt it yourself. You will find the RAM slots in laptops in one of two places: under a small cover on the bottom of the case, or inside the case on the main board under the keyboard. If you have no experience disassembling laptops, I can't recommend pulling the keyboard to add RAM. Very often, however, you can add RAM easily. Turn the laptop over, identify and remove the cover for the RAM slot. RAM sticks for laptops are shorter and wider than desktop RAM, almost square. The RAM stick is held in place by tabs at each end. Spread these gently and the RAM will spring up from its slot and will rest at an angle. Pull it straight out to remove. Replace with a stick with greater capacity. Insert the RAM into its slot at about a 45 degree angle until you can insert no further and can no longer see the gold contacts. It is keyed like desktop RAM, so if it won't insert, flip it over. Like desktop RAM, you need to add the right kind and the desired amount.
8. Now for the moment of truth. Power on your computer. If you hear a series of beeps, and the computer doesn't boot, you have a problem. You have the wrong RAM, or it is not seated well, or you have knocked something loose. Go back over each step. Look carefully to make sure nothing else is loose. Make sure everything is seated well. As a last resort, put the original RAM back the way you started. If it boots, you have the wrong RAM. Take it back.
9. Normally, replacing RAM goes smoothly. Check the task manager. Make sure your system recognizes all the new RAM. Your "Available" number should be much larger. Enjoy your improved performance.

As originally stated, these steps assume you have a bit beyond a rudimentary knowledge of your hardware. If you cannot open your case without assistance, leave these procedures to seasoned friends and relatives, or pay a technician. But if you need RAM, get more. It is worth it.