Post -vs- Get

I have written a system with authentication to gain access to individual data. Since I am using the post method the command line has something like abc.com?client_no=5 Now even though if that person emails you that url, you won't gain access to it bascause of authentication that is needed, what can happen is an authorized user could change the 5 in the url to 8, and then see the information that is there which belongs to client 8. Because of this I am thinking in this case I should be using the post method instead of the get method. Does anyone have a link to a doc that has a good example in both methods along with a good explanation so that I can make sure I understand what it's doing? Thanks for your help! Jack