I just taught my wife dominion. You need to start with the base set recommended cards. Play a handful of games. If your friends hasn't played a deck builder they need time first to understand the concepts. Also it's helpful if you play face up. Tell them exactly why you're doing what you're doing. Remember you have probably hundreds of games worth of experience and they have none.
I think next add seaside because duration cards aren't too bad, they aren't complicated. Add alchemy last. Maybe add prosperity after seaside. It has a lot of fun cards. KC is really fun card to play with.
If you want to add more variety, here's a game I designed myself for beginners: King's Court, Council Room, Lab, Tactician, Thief, Worker's Village, Treasure Map, Look Out, Warehouse, Light House. It works for both a Colony game and a Province game.
I like the game because it adds a lot of fun cards and some simple cards to show some concepts. It has plenty of buys but there will be turns where they can only buy one card. It has a really tame attack but if you KC that thief it will feel like a much worse attack especially to a newbie. Then light house of course can prevent that attack. It has +action but again you can still draw dead actions because you won't be able to always have the WV. It can definitely be augmented, but I've had success with this.