The Julii have the easiest access to land out of the three factions. All that stands in their way are the Gauls, brits, and germans.

On your first turn you should get a mission to take Segesta, so take it garrison it with some troops, and move on. On your border should be a small Gaulish army, on turn two attack. Some people may some wait but the longer you wait the stronger they become so you have to move fast. Take Mediolanium then first, that will cut off pativium, then take pativium. Doing this will really hurt the gauls economically and give you good cash.

For the homefront concetrate on building hastarii for now, while working on getting stables, for cav.

Ok, after you've licked your wounds from taking the first three provinces, move in and take masilla. It's rebel held so it shouldn't be so tough. After that, move a part of your army towards lugdunum, it also should be rebel held. Now you're have the gauls by the threat, and this is what you should do:

If you see any gaul family members around bribe them, you should have enough money if you have a good economic plan(I won't tell you how to build, except to concetrate on getting markets, roads, and stables built,) and after you bribe them, have either a relief army join them or have them hire mercs.

Now if you bribe one near Narbo Martus have him move in on it quick, it should be easy to take, but be careful, the Gauls most likely have a big army in numantia.

For Alesia, get your best army and move in on it, expect a tough battle, for it's the gaulish capital. Once you take it the Gauls should switch their capital to numantia, so get your army in narbo, make your general hire some mercs, and move them out, (making sure to leave some TW in narbo). Your army may encounter a few large Gaul armies so take them out. Don't let them escape. They also may have a fort on near the bridge leading to the town so take it out. Once you reach numantia, lay siege to it, attack it when you're ready, and the gauls should be finished. Take over the rest of their lands if they're rebel or gaulish,(if they survived). It took me 28 turns to defeat them and this is a very rough guide.

PS: Don't forget to try yo bribe Caralis from the carthagians.

EDIT: Guide to take on the british and germans coming up after I beat them.