I have a 99 forster it has a TC1A37C2AA trans in it. I have stock trans from a parts car. Do I need the computer from the car? Also is there a gear difference? I'm new to these cars. The front differential is bad in mine.

No you don’t need the computer. 

A different computer won’t have the right pin outs so that’s positively not what you want.

The computer is meaningless for what you’re trying to do, ignore it. Match the trans to the vehicle (aka the trans needs to the computer and wiring).  For what you’re doing, you can not swap computers to make a trans compatible

1. You need a transmission compatible with the existing vehicle (aka computer and wiring) - you can’t swap computers without gutting the entire vehicle and doing a full wiring conversion with it.  You match the trans to the vehicle - so any 99-03 Forster or 00-03 outback.  Maybe even 04s  

Those vehicles match the wiring, TCU and final drive ratio of the original rear diff in the vehicle.  

Determine your current rear diff final drive ratio and tell us what it is. Should be 4.44, which is what I think all those other transmissions are but check first. There are.a few oddball final drives around 03 but I forget which model and years exactly and maybe it’s only manual trans