Whether you will be able to do want you want will depend on whether the type of modem that you have will allow the "guest" network feature to work correctly on your Time Capsule....and....whether the model of Time Capsule that you have will support the "guest" network feature.
So, what model of Time Capsule do you have? Is this a tall or "tower" shaped Time Capsule, or is it a flat or "square" shaped Time Capsule? If it is a flat, square model, we need the model number from the device. It is usually embedded in the foam base, and it starts with an "A" followed by 4 numbers.
Next, we need the make and model number of the "modem" that you are using. We ask, since most modems provided by Internet Service Providers will not allow the "guest" network feature on the Time Capsule to work correctly.