While most Macs ship with between 4GB and 16GB of RAM, sometimes the lower end of this range may not be enough for your uses. If your system is running low on RAM, you will experience slowdowns when launching programs, loading documents, and otherwise using your system.
If your system is low on RAM, you should be able to see this reflected in the Memory section of the Activity Monitor utility, where in OS X Mountain Lion and earlier there is a small pie chart showing the amount of free RAM vs. that reserved for system processes. If the green wedge in this chart is constantly below about 25 percent of the chart's area, you might want to consider upgrading your RAM.
In OS X Mavericks, the Memory section in Activity Monitor shows a Memory Pressure chart instead. Green indicates that your usage does not exceed the system's memory capability. If this chart's numbers are regularly high, you may need to upgrade your RAM.
First, you should determine if your system's RAM is upgradable. In essence, if you have a Retina MacBook Pro, or a MacBook Air, the memory on the system is soldered to the motherboard and cannot be upgraded. Otherwise, you should be able to upgrade your RAM.
Next check the RAM's type and speed, which can be looked up by choosing About This Mac from the Apple menu. In here, note the speed, which will be something like 1,333MHz, and the type, which will be something like DDR2 or DDR3.
Now you can purchase new RAM for your system. While 4GB is a general minimum, if your system can handle it, then install at least 8GB, but more is preferred. For computing, in general the more RAM you have, the better.
On some systems, all available RAM slots will be filled, so when purchasing you will have to consider replacing the current RAM chips, but on others you might have open RAM slots that you can add new RAM to. To see this, open the System Information tool, and choose the Memory section, where you will see the available banks of RAM and the size of RAM chip installed in each.
If you are uncertain how much memory your system can handle, then the following Apple knowledge base documents should show you the capacities of your system:
These documents also contain a number of images outlining how to install upgrades on your system, but in general, once you have exposed the RAM slots per the above articles' instructions, you unlatch and remove the old RAM and then fit the new RAM in its place.
While you might be concerned about whether RAM is seated properly, keep in mind that you cannot easily damage RAM or your system if it is not properly seated. At most, when your system boots up it will not be able to detect and test the RAM, and will issue three beeps without booting, or will boot and not show the RAM as being available.
Even though improper seating cannot easily damage RAM, do keep in mind that static electric discharges to it can damage it. Therefore, when opening and installing new RAM, regularly touch the system to ground yourself to it, and use a static-free surface like a wood or glass desk.
Keep in mind when purchasing RAM, that your Mac contains the same components as any other PC, so you can get any RAM that matches the specifications for your system. While some vendors may try to sell Mac-specific RAM, you can use any that matches the requirements for your system. In doing so, you can avoid places that sell "Mac RAM" at a premium.
After upgrading, be sure to run Apple's hardware test suite to check the new RAM for errors. This is perhaps the most important step of any RAM upgrade, since corruption in RAM can result in crashes, hangs, and data corruption. In addition, RAM errors can go unnoticed for a while, and then suddenly start to affect the system.