General advice:

1. Clean and prep the board with isopropyl alcohol and allow to evaporate.
2. Use flux, I quite like the no-clean flux pens for applying this.
3. Clean and tine the tip of the soldering iron.
4. Set the correct temperature for your solder.
5. Heat the joint (not the solder) by holding the tip of the iron to it for a few seconds.
6. Feed the solder to the joint.

These are approximate temperatures all around +/- 10C so you may need to play around with these to find what works best for you. I use a 40/60 lead/tin solder and I currently have 0.7mm but when I run out will switch to 0.5mm if I can, for this I find 345C works for me.