Build an engine with +cards and villages if both of those are on the board and get at least one copy of the best way to permanently get rid of your starting cards on the board as early as possible. Once you're drawing your whole deck every turn, you have a lot of different options depending on what's available and what your opponent is doing; you could keep building a bigger engine that buys multiple Provinces per turn (which tends to be kind of hard to do and unnecessarily slow in just Base + Seaside, but it is still sometimes worth doing especially if you can just build a 1-Province-per-turn engine and add an Outpost to it to effectively make it two), build up to 1 Province + something else per turn which can keep your deck functional while you're greening and possibly let you get Province + Duchy turns later, or just immediately start buying Provinces if you're in a desperate hurry or if your deck can easily handle adding that many green cards to it. Sometimes you have to try to get away with an engine that doesn't consistently draw the whole deck every turn, or perhaps can't quite draw the whole deck at all.
Some attacks like Militia, Blockade and both types of Witches are worth buying early, and some others like Corsair and Bandit can add more value to your deck when you're playing them every turn but aren't critical to get early unless you don't have anything better to do.
If you can't build an engine, usually the best strategy is to buy a relatively small number of the strongest-by-themselves kingdom cards and just basic treasures and other cards like Sailor that boost your tempo without antisynergizing with anything else you're doing.
There's a lot more nuance to it than this but this is the very short and mostly basically correct version.
