Beginner Power Grid strategy
There really isn't a set strategy here, but a number of tips I've found useful from playing Power Grid, and I hope they can help you, too:


Look at what everyone is buying -- if you're playing 3rd in a 5 player game, and the first two players bought coal plants, consider going with a different fuel source, as coal could get expensive quickly if everyone is buying it.  Pay attention to how quickly fuel sources will replenish at the end of the turn as well.  
If you are first to buy resources, consider buying the maximum amount.  It may cost you a little extra to buy them, but anyone else needing those resources is going to have to pay that much more.  Feel free to put the squeeze on people this way.
Don't overpay for Enrvironmental plants -- the environmental plants (the wind plants) are definitely a good buy, and everyone knows that.  If you get lucky and can buy one at the end of the round for face value, go for it.  If the #37 shows up and it gets bid up to $60 or $70 quickly, think very hard about how much you'll be saving if you buy it.  They're good, but don't overpay for them.
Beware the East coast -- there are lots of cheap connections in the East coast, which is great for expanding quickly, but if you buy lots of houses quickly, you'll be in first position, which means you'll be going last lots of the time.  Also, with cheap connections, people might crowd around you and block you in, meaning you may have to expand two or three hubs away, and pay some steep connection costs.  I personally like moving into the East coast during phase 2.
Nuclear plants are great later on -- early on, nuclear fuel is very expensive.  Within a few turns, however, the fuel can get down to $7 or $8 for a single piece.  Compared to how much coal and oil may end up costing, nuclear plants are good for mid- to -late game.
Time when you buy cities -- sometimes there may be a good plant on the table coming up, and you want first shot at it.  If everyone else is buying cities, consider staying at a lesser amount of cities so you have a better chance at going first.  
Don't buy plants for the sake of buying plants -- If a good plant comes up, feel free to bid on it, but don't feel like you have to bid every turn.  Try to buy as few plants as possible.
Have a long-term strategy -- if the game will end when someone has built their 14th house, figure out what combination of plants will best get you to 14, such as a 4/4/6 setup, 4/5/5 setup, 3/5/6 setup, whatever.  There are some fantastic plants mid-game that you can use for the rest of the game.  


If I think of anything else, I'll add them here.