How do I ensure my WLAN is secure?

Require authentication for your WLAN. When setting up the router, it usually asks if you want to put authentication or not. With authentication though, you will have to enter a login and pass to connect through your own network also. Without authentication, anyone with WiFi enabled hardware who passes at least 20-30 feet from an access point in your house can piggyback on your internet connection.

any type of lock or security system only serves to keep the honest honest- someone who is really determined can hack you regardless how much time, effort and money you pour into the thing. wireless is by it's very nature subject to hacking because it is a broadcast system- anyone with the right radio equipment and knowledge can get into your system or anyone elses- for that matter having an internet connection subjects you to the same risks- if you absolutely must have a totally secure system, unhook them all and throw the suckers out- you can't hack a non existent system.

make the router only connect to sertain mac codes which can be found on the usb wifi adapter or the wifi card. and use at least wep security. if you arnt computer knowledged then get someone in to sort it out.

In the set up of your wifi it would have ask if u want encription, use that. I use 128 cypher encription

The only way to be sure it is secure is to unplug it.

have it encripted with a password