Check what will score
The first thing you should do once you've been given your tiles is to check what the scoring permutations are on the current round. You can do that by tapping the markers along the right hand side of the screen. This will show you not just what the tiles in your hand are worth, but what your opponents might be aiming for too.
While it's important to know what's going to be getting you VP this turn, you should always check what the different scoring combinations are in the future too. You'll often score more than once for a particular set, so it's a good idea to focus on those when you start in order to bag regular points throughout the game.
Don't go broke
It's tempting to set the prices of tiles you want to keep incredibly high, but that can have the undesirable knock-on effect that no one buys them. That's going to leave you with a steep bill to pay, and less room to change tack. In the early part of the game, give everything a reasonable price, and save your extreme prices for later on.
Connect the pieces
You'll always score at the end of the game with connected bits of pasture, sea, or mountain, so even if you're not scoring with them in a particular round, it's a good idea to place your tiles so that they join together. Single tiles that are complete in and of themselves are great for this, so watch out for them.
What are your opponents doing?
Every turn you'll see what tiles your opponents discard, what tiles they purchase, and how they lay them. This will give you a good idea of what they're aiming to do. You can also keep track of what they've done by tapping the history button.
Try and make it as difficult for them as possible by pricing them out of cards they need, or sending them back into the deck so they can't be bought.