You only need a bit of gold to do this, preferably around 1500-2000. Firstly, finish the Companions' quest line and get Aela The Huntress as your follower. Then, ask her to train archery and do it 5 times. Trade with her and then you'll be able to get the money you invested back. Rinse and repeat. Note that you can only train 5 times each level, so do it every time you gain a level.