While the 30 watt iron would be okay, if you aren't too sure about your soldering skills on the small parts I would suggest you pick up a 15 watt iron instead - you will be able to leave the iron on the board longer before you run into problems.
Just touch the tip of the iron so it touches the board and the pin on the pinheader, apply your solder, and remove the iron. Make sure you have applied a good amount of flux first.
No matter how much easier you think it looks if you apply the solder to the iron then touch that drop of solder on to the pins, don't do it, it isn't going to work how you might think.
On the other hand, for the underside D0 point you can get away with that if you have a low wattage iron, it may not be the worlds best solder point but it seems fine for that one connection considering it is the hardest point to make for a first time solderer.