As long as you don't have file sharing turned on (or Windows file sharing), then they cannot get access to your Mac. If you have sharing turned on, then they'll need to figure out your username and password in order to gain access to anything.
Also if you don't have printer sharing enabled then they won't have access to your printers either.
They will be able to tap on to your connection if you don't have any kind of security enabled. The best security is MAC Address filtering if its available for your router. This will ONLY enable users to access your network if you've put their MAC address in. Every network device has a MAC address (FYI: Networking term MAC has nothing to do with Macintosh, or Apple itself), its usually 2 numbers/letters separated by a colon. This is arguably, the best way to secure your network. If someone wants to sit out in front of your house and try and spoof a MAC address to gain access to your network tell them good luck!
