You need more RAM. With only 1 GB of RAM, the OS is leaving little RAM for apps & it is swapping out memory much to often. Looks like this model can access 2 GB, so fill in the second bank with another 1 GB module. When buying memory check that it works with your model, most sites offer products by model.
Also, go to System Preferences > Users & Groups > Select your user name > Select Login Items tab & remove any user apps installed, this will eliminate overcrowding of the RAM at startup. Only run apps that you are actively using
That should help