Not enough information for a solid answer.
Is the router supplied and set up by your provider? If so, you may need to contact your provider and find out if and how you can add the iPad.
Or did you buy, install, and set it up yourself?
In that case you need to go back through the setup and make sure you enabled DHCP service. This is the service that allows devices to automatically join the network. Also note the the name given to the wifi network, security (if used) and password (if any) needed to log on.
If you enable security [recommended] you will need the password and security type. If you also hid the network, you will need to remember the network name.
Go to Settings >> General >> Network >> Wifi: If you can see the network, select it. If the network is open, you are in. If it requires a password it will ask you to choose the security type, then a password
If you can not see it, choose Other. You will need to enter the networks name. Then it will ask you for security type and password.
If none of this helps, then you probably need help setting up the router. There may be a setup wizard or other tool that you can use. Look for it and follow the instructions. If not look for a manual, probably a pdf on the CD that came with the router. It should have an easy setup section. You can also check the manufacturers site, and or forums for help.
That's about all I can do. Each router has it's own setup and options which makes it very difficult to trouble shoot problems remotely.