What the others have said but also.
Don't confuse hard disk space ( I've got masses of memory left, I've got some 214GB left out of 320GB, ) with RAM. Enough RAM is much more important (most of the time).
Go into either your Utilities folder or Applications folder and find Activity Monitor. Fire it up and first of all go to Window > CPU Usage. You should see a wee little box appear with bars in it. When you have the 'Spinning Beachball of Death" have a look at those bars...have they nearly filled the column? Let us know.
In the main Activity Monitor window select CPU...that will list a whole load of things. Click on the column %CPU (or similarly named...Apple tend to change the name from time to time). That should sort those list of things depending on how much CPU they are using. When you get the Beachball, you should see a few items right at the top that are consuming all those CPU cycles. Again, let us know the name of the item. You may also see some items in red. Again, let us know the name.
Next, select Memory. Right down at the bottom is a table saying things like
Go up to your Apple logo top LH corner and click on it. Click on About This Mac and tell us what the numbers are underneath the OS X. Version what? How much Memory do you have (says it lower down) ? There will be labels like System, User etc. Take a screenshot of that area and post it on the forum.
How to take a screenshot of only a certain area? Easy. Go to Preview > Take Screen Shot > From Selection.
