Tribals can build temples. Click on the empty slot and if you have the gold you can build it. Get said gold by raiding.
To raid, raise an army, select it, and press the toggle looter button. It's located between the 'split of special troops' button and the attrition indicator. This has to be done on your territory. Then either park the army on neighbours, or put them in boats and ferry them over to whatever coastal province you want to raid. The boats have to be next the province you put the troops on to raid.
Every province has a loot meter. When hostile troops are in the province, the meter goes down, which affects taxes for the owner. If you are raiding (looting is the same thing), this gets translated into gold and prestige for you. The level of fortification in the province will decrease how much of this loot is easily available. To get to protected loot you have to fully siege down the holding. This can also get you some fun events, traits and prisoners. However, the most efficient way to raid is to have a small army of about 500 troops and just go for the unprotected loot.
If you keep your troops moving, you can usually get most of the unprotected loot before the troops get to you. Having an organizer would also help you outrun any defenders. If you raid small realms (like the Papacy in the 867 start) that only have one province, the ai won't raise its troops while you are besieging its county. These make good targets for sacking. The Papacy, Venice, Amalfi and Irish minors are examples of these.
My general approach to reforming is to beeline for the holy sites in Scandinavia (either with subjugation or forged claims if necessary) and see where that takes me in terms of moral authority. If it's close, sack temples of other religions or build temples (preferably both). Most counties in Scandinavia don't have temples to start with and remember you can build in your vassals territory. If you need way more than that, look for oppurtunities to snipe Zeeland or Brunschweig (or wherever Paderborn is) via county conquest CB. This is somewhat dependent on what the Karling blob is doing and how messed up by Gavelkind it becomes. You could also try blitzing county conquests on 1pms in Finland or Britain/Ireland. Also, a successful prepared invasion will net you 10% MA IIRC. Once again, Britain is a good target for this, but make sure you can actually win it. Losing costs you 10 MA and other Catholics will likely join in to defend a coreligionist from invasion.
If all this fails, you could also just go Catholic (but stay Norse cultured!) so that you can go Feudal and get out of the hell that is Elective Gavelkind. Stay Norse so that you can continue to raid (yes, even as a feudal Catholic) and get cheap ships. Norwegian/Swedish/Danish/Norman have to pay full price for boats. If you don't get to excited about converting your provinces, courtiers and vassals, it isn't even that hard to go back to Germanic later.
