Solder does not stick to tip of soldering iron?
First question: How long have you had the tip? You could probably easily and cheaply replace it. If the iron doesn't have replaceable tips, get rid of it!

If you've not had the tip long, then a few pointers to keep in mind:


When the iron is not being used, keep some solder on the tip. This prevents oxidation and corrosion.
When using the iron, keep a little solder on the tip when the iron is in the stand. Wipe clean before using. Avoid excessive wiping on a wet sponge as this can cause temperature fluctuations which expand and contract the metal, and stress the tip. Consider using brass shavings instead of a wet sponge.
Never use sandpaper or abrasives to clean the tip. Also avoid dipping it into flux to clean. (Except for re-tinning which should be infrequent.)
Use a good quality solder. Old solder can have contaminants which leave behind residue and basically make soldering more difficult. (By "old" I mean solder that's been around collecting dust, not necessarily that it has a short shelf life.)


Finally, if you've got a cheap iron, it may be too hot or cold and not melting solder at the correct temperature. Consider getting a temperature controlled unit. I could recommend a Hakko FX-888D for ~ $100 US, see also this tutorial.

Some additional soldering tip pointers:


http://www.inlandcraft.com/uguides/tipcare.htm