Can someone tap into your WiFi?
If you use WEP - then yes, and you, sir, are reckless. Switch to WPA/WPA2.
If you use WPS and have a router older than 2012, then yes, and it will be brute-forced. Just disable WPS. You only used it a few times, anyway - admit it.
If you use WPA2 and a weak passphrase, then yes, and it won’t take long. Generate a random key to use for WiFi, and check it against popular password lists. “Rockyou” will be a good start.
If you have disabled WPS and use WPA2 with a strong passphrase, you are decently protected here. Focus on other weak links. For example, take the post-it with your password off the screen ;-)