My recommendations, YMMV:
1. 15W is good for SMT.
2. Get an iron with a ground (earth) lead on the plug. I have a cheap 15W Radio Shack iron that works fine.
3. Heathkit had a wonderful soldering guide with each kit. They're gone, but here's a decent site I found via Wikipedia: http://www.solderinguide.com/
4. Use the iron to heat both the pad and the component, but don't touch the solder to the iron's tip -- let the component and the pad melt the solder.
5. Keep a damp sponge handy for cleaning the tip.
6. Re-tin the tip frequently.
7. Gertboard has a fine-pitch device that's going to be a pain to solder. You'll probably need solder flux and de-soldering braid as it will take a few tries to get that device soldered. At my age I need a stereo microscope for anything under 50 mil pitch.
7a. My approach to that nasty component on Gertboard would be (1) get some solder on each pad, (2) squirt some flux over the pads, (3) place the component over the pads, (4) heat each component pin in turn pressing down gently, reflowing the solder already on the pad -- the flux should help the solder flow onto the pin and help prevent solder bridges. You'll need a really small tip for that component.