Welcome to the Haven campaign. This is fairly simple, so it's a natural starting point in the game, even though you can start with any campaign you want.
A natural strategy of the Haven faction is unit conservation; trying to fight battles with as few losses as possible, notably through the use of healing spells and units, but also defensive abilities. Haven units are tough, and those that aren't protect the ones that are. I recommend giving your Haven heroes (the main hero in particular) plenty of attack and defence bonuses, so as to boost the creatures passively as much as possible.
Let's get started. You begin with Tomas of Wolf, and the first map is fairly easy - it serves as a good introduction to the game's mechanics. Walk around, fight everything and kill it while trying to get used to how the game works. Try to sustain low losses if you can – the autocombat does work well but if you lose too many troops, perhaps you should re-fight the combat manually. Also, some troops may want to join you. Free troops! Yess. You are guided through it all and should not have any problems exploring the map once you learn the difference between ornamental buildings (annoying, I know) and functional ones.
The skills you should choose for Tomas include those useful for defending a town. Also, having diplomatic skills increase the chances of getting neutrals to join your army.
The main quest is to clear the Falcon road, and to do that, you first need to rebuild a bridge. That costs you 500 gold, 4 ore and 8 wood. Walk up to the bridge, click it, and watch the magic happen. Keep marching and clear the roads of bandits and brigands. Basically, just kill everything.
Eventually, you reach a big town, which is defended by your main rival. If the threat predictor says you should win easily, go for it. Fast, before he can buy reinforcements. If you can't attack, pick up reinforcements from the dwelling on the other side of the water. Once you have won the town, build it to maximum fortifications and get as many creatures you can. Prepare for invasion - there is one coming. If you use your time well, the invading army should not win the battle when attacking your town. Winning that battle wins the map for you.