Newbie question - how do I build things?

Just started playing Fallout 4 - really enjoyed the TV show. Never played any of the Fallout games before.

I have met the minutemen, and we have all gone back to Sanctuary. I understand I need to build things, but how do I do this? If I go to the workbench and try to build a structure, it says I have all the raw materials I need, but will not let me build - it's greyed out.

How do I build things?

Thanks in advance