Thanks, I've been trying to get county control but it's taking too long and I'm getting constant invites to wars before my Marshall completes the first control. Does it make a difference if I decline wars? Or shall I not create so many alliances?
I played as Halfdan not too long ago and I found out (like you) that the alliances are counter-productive. At game start, you have the extra levies so you really don't need allies to deal with anyone around. My advice would be, don't form any alliances and after you win your first war, keep the 3 provinces in Duchy of York (Jorvik) - they are all plains (iirc), and can provide decent income early on.
Then just go raiding. In Wessex, Canterbury can give a lot of gold, try to target that first. Then, keep an eye on powerful realms who are at war, and raid around their capital - those are usually rich. You can sail right up to Paris, and if the French king is at war god-knows-where, he will never march back in time to stop your raid. The key is to not be too greedy, you don't want to get caught out and lose your army. Raid, bail, bring back gold, build income buildings, repeat
Once you get more experienced, you will be able to judge better the times when getting caught is desirable to fight battles for prestige. But if you're new, just focus on bringing the money home