What you’ve got there is an image imported to sketchup. These can’t be altered or dimensioned as you’d like to be able to within the program. However, you can scale the imported image by selecting the image in question and dragging the top right corner so that one of the walls you know the dimension of matches what you have in real life.
From there you can trace around the floor plan to create your wall, push/pull them up and then add in details.
It is all quite overwhelming to begin with but fairly easy to get to grips with. But take heed of SU moderators suggestion of following SKetchUp School’s tutorials and start from the very beginning. It is easier than it appears and you’ll be modelling your house in no time.