Hello BaruchB,
I am Jaspreet Singh.
Please confirm with your internet provider what the username and password is for.
To connect to wifi you simply need to click on the wifi icon on the desktop then click on the network (new one in your case) then enter the password for the network.
You do not need a username in such case.
One reason might be if your network provider has an open wifi network but requires authentication for logging in to the network after connection. In such a case simply connect to the network via the above steps to the wifi network your new provider has set up then it will open a webpage where you will need to enter the username or password.
You do not need to clear any previous wifi networks etc for connecting to new wifi network.