Er... ok... This is for PS 5.... sould be around the same for the other versions... make sure you have the "options" window open. If its not, click:
Windows -> Show Options
First click on the Rectangular Marquee tool to select it
Then, at the options window, click on the Style drop box and select constraint Aspect Ratio.
Then, enter the required ratio into the Hide and Width textbox.
Now when you select, it'll be fixed at the ratio you provided.