The road network in Northern Norway varies from silky smooth and well-signposted, to... well... interesting. However, all the roads get you where you want to go in the end, and it's generally easy to find your way. Take some detours onto the smaller roads, follow them to the end, and see where they take you!
High season and low season
There are car rental companies all over Northern Norway, from the familiar chains to petrol stations that have a few cars to rent out. In high season from June to August, however, cars are in great demand, and it's better to book in advance. Outside the high season, however, it's never a problem getting hold of a car, and you can often get a good deal, particularly over weekends outside the high season.
Don't hire a car in one location and drop it off somewhere else – someone then has to drive the car back for you, and that costs money! If you are visiting several regions in Northern Norway, it can be worth renting a car for short periods in one region, then using public transport from there to the next region.