Any chance you can show us a pic of the iron your using?
You want a medium/large sized tip as the bigger the tip the better it will transfer the heat to the wire (more surface contact)
Leaded solder is easier to use but if you your using a decent iron (Antex are really good though you pay for the quality) high wattage 60W then lead free is fine as well but harder to work with (Electronics solder not leadfree plumbers solder
it will have a good percentage of silver/tin content)
Like everyone says tin the iron as it heats up (this bits important) then tin the motor tabs and wire, a small amount of flux will help as it cleans the surface for you (again electronic flux not plumbers flux as it can corrode electronics, so I'm told)
then put heat to the tab and wire together add solder to it, should run smoothly on then remove iron, this should leave a nice shiny joint. wipe the flux residue off afterwards and wipe it off your iron as well using a damp cloth.
Like someone said most solder comes pre-flux (not all) but a little extra can help. but like anything practice makes perfect.
Anyway hope that helps and best of luck and if you get stuck ask someone at the local track but check theres first
