There's a few ways to do that depending on what you want to accomplish.
If you have a series of surfaces that make a closed volume and are not yet knit together, you can use the surface knit tool. Select all of the surfaces and once you select the last one that closes it off, you will see an option in the property manager "Create solid". Check that box then hit OK. You will have a solid then.
If the surface is already knit a closed, then you can use thicken command. Check the box to "Create solid from enclosed volume"
If you have an open surface, you can thicken that into a solid as well by using the thicken tool. This will add thickness to the surface by offsetting it one way in either direction or equally in both directions depending on which option you select. Enter the thickness you desire in the property manager.
Regards,
Jon