Soldering is all about heat. Both surfaces (pad and wire) need to be sufficiently heated before applying solder. Put the wire through the hole, and touch the soldering iron so that it is heating both the pad and the wire at the same time. Give it about 1 or 2 seconds of heat, then touch the solder to the pad - it should wick up onto the wire as well. Don’t heat longer than 10 seconds. If you’re having trouble, just take a short break to let it cool (say about 30 seconds to a minute), and then try again.
There are a lot of resources around. Google “soldering howto”