Computer Problem

Our Mac's computer memory is not even that full but it seems our computer is still slowing down. We're not really sure what the problem is. Once it starts slowing down, we just restart the computer but it's no longer that fast anymore. The laptop we gave to my sister has died also. I feel bad. Hope there is a way to fix it. She said that not a lot of people own a mac in the Philippines and she think it's very cool.