Strategies and kingdom cards for a strong opening in Dominion?
Good cards to buy early:


Chapel: this is close to a must-buy if it is on the board.
Sea Hag: another almost must-buy so that you don't fall behind on curses. It's less valuable in a multiplayer game (because if everyone else is firing it, the curses will run out quickly, and then it just takes up space in your deck).
Ambassador: functionally trashing your deck while bloating your opponent with bad cards. Ambassador/Silver/Ambassador can be a devastating opening. 
Militia: especially in 3+ player games, this is brutal. Less effective if a Library is on the board.
Moneylender: trashing + accelerating you to 5/6 coin cards. But, it can potentially clog your deck in the mid/endgame.
Envoy / Smithy: these cards accelerate the entire game. Big Money buys 4 provinces in 17 turns, on average. Purchasing one of these cards lowers that to 14.5 turns.
Cutpurse: also deadly in multiplayer games, but still great for slowing your opponent early. Less valuable as an attack late in the game, but it's still worth a Silver if you have actions to burn.
Trading Post: if you open 5/2, this is a great card because it trashes your junk into silvers, which also allows you to save your initial buys for non-treasure cards.
Mining Village: trash early for a 5/6 coin hand.
Baron: useful early for an easy 4 coin. Can clog a deck later in the game, but its ability to give you an extra Estate and an extra buy can sometimes tip the balance in a close game.
Loan: useful for trashing coppers without costing an action and can also be used to cycle your deck, getting new cards shuffled in faster.
Quarry: useful if there are expensive actions in the set (King's Court, Forge, Peddler, and game-changing 6 cost cards like Goons).
Pawn: there are many times where the +buy or +coin will be useful; in the worst case scenario, it's a self-replacing card (+1 card/+1 action). Because it only costs 2, you probably want to buy it earlier instead of wasting a 7 coin hand on it.