It's nice that you want to help but in this instance, you're not really helping.
The OP is using a Mac. Mac's don't have "command prompt" they have a "terminal" window.
New Mac's have a BSD (that's UNIX) based operating system and as such don't have "ipconfig". They use "ifconfig"
I'm not a Mac guy so don't ask me where to find a "terminal window" but if you know, or can find it on your own, open it and do the following command:
In the output look for the Default Gateway address. That is the one you'll want to enter into your web browser to access your router's management interface.
Once there, you can then secure your wireless network. I'm guessing you didn't use encryption and a passphrase and that's why you have neighbors using your WLAN. Enable WPA encryption and use passphrase (key). Then the next time you connect to your WLAN with a wireless client, you'll need to enter the passphrase. Anybody who doesn't know it, will no longer be able to access your WLAN. Use a complex password for the passphrase.
Also, you can specify which wireless clients are allowed to connect by their MAC address if your WLAN is already secured or whoever's using it figures out your passphrase.
It matters not how straight the gate,
How charged with punishments the scroll,
I am the master of my fate;
I am the captain of my soul.