how to transfer app from computer to iphone?

Connect the iPhone to your computer and select it on the left-hand side of your computer's iTunes, and on the right-hand side there should be a series of tabs, one of which is Apps - if you select that tab then you should be able to select which of your apps to sync to the phone (assuming that the apps are compatible with the phone e.g. you have the relevant IOS version on it). If you have bought/downloaded apps on your phone that aren't on your computer's iTunes then you may want to copy them to your computer's iTunes first via File > Transfer Purchases

Syncing apps : http://support.apple.com/kb/HT1977 (the page isn't maintained anymore, but it seems to be up-to-date).