What do you think of first, making words out of the tiles that you have or figuring words that would fit into the highest point squares?
A bit of both. There are a lot of things to consider depending on the board and your rack. I guess I think about a mix of the following - in no particular order :
1. Look for bingos
2. Look for coloured spaces that can be used in two directions for high scoring letters (don't waste a high scoring letter or a useful letter like S just on its face value)
3. Improve my rack if there's good potential (including space) for a bingo next turn (eg if I've got a 'satire' word plus Y - dump the Y somewhere)
4.Block potentially high scoring spots if I can't use them myself - including places to use high scoring letters if I know my opponent has them (eg blocking the only open I on the board if its near the end and I know my opponent has the Q)
5. Try and get more points out of high scoring letters already on the board.
6. Close up the board if I'm well ahead so that my opponent can't come in with a bingo and beat me.
7. Etc etc
Just some thoughts. But I lose a lot .... watch some games on the net between high ranking players and you'll get some inspiration.