I'm still a new player, but here are my thoughts on the game. Note that I refer to house/watchtower/castle as buildings, sheep/tree/axe/gold as resources, and tiles as the combination of the two.
There are two main goals in the game:
1- Getting the buildings needed to complete your port/altar/palace.
2- Getting ~3 high scoring tile types
For the first goal, you should look at your board at the start and see which buildings you need the most of. 5 houses, 3 watchtowers, or 2 castles are all notably high amounts that will require a specific focus on getting them. Really, you'll need to focus on getting the exact amount of buildings you need. Don't be afraid to grab them away from other players or spend your shields to get them. At the end of the game, nearly all buildings should be taken and you should have your exact amount plus maybe 2 more.
For the second goal, you want to maximize the multiplicative scoring to get big returns. You should aim to have around 7-8 longships for about 2-3 per type, with a total of ~25 points per tile type for ~3 tile types. This can be a number of methods, such as 5 sheep for 5 points each, or 2 castles for 11 points each, or 4 gold for 6 points each. Of course, don't be afraid to get more tiles or longships that are already high scoring. The main limit here is the lack of good tiles to place on and same tile scoring longships.
Speaking of longships, you might be wondering which ones to get at the start. Simple: You should get longships for buildings you will have a lot of (see above), or barring that, just whatever longships that other people aren't getting. Early on, you have the flexibility to pivot to whatever resources you need, so any longship is fine. Even a single black longship is good enough. Additionally, don't worry too much about what resources it needs. You should look to overlap as much as possible, especially later on, but the point scoring is far more important in general.
Finally, a note on axes. 5-6 axes can replace a high scoring tile, but you'll want to still to get the same amount of longships to make up the point difference. Your points will still end up the same, but will be more spread out.
Tallying the above, we get 21 + 25x3 = 96 points. Add in miscellaneous extra points from other tiles and you can easily crack 100. A particularly high score is 120, which requires maximizing your gains but is still possible to achieve.
