I usually start like this:

Economy:
- disband your fleet, you won't need one until much later
- set your taxes to very high
- from your starting treasury order roads everywhere and queue up a river port in Bibracte, this should improve your economy a bit
- you will go into the negative for a while, but should be able to make a profit after expanding

Military:
- do not disband any troops, you'll need them
- use the troops in Cenabum and those with Dennoros to besiege Avaricum, wait out the siege because it's strongly garnisoned
- move the troops south of Gergovia through Eleutheroi-territory to join the siege of Avaricum
- move the troops in Bibracte and Mediolanum towards Viennos, besiege it or try to take it with a direct attack (use your spy!)

After a few turns you should have taken Avaricum and Viennos. You can now combine what is left of your troops and conquer the other Arverni-towns, either simultanously or one after the other, depends on how strong you are.

After the Arverni are destroyed you should be making a profit again and get out of the debt. If you have any troops left you can try to expand further. For your economy it is best to conquer Aventicos and the two alpine towns, for they have mines.