here a tutorial to make a frame.
start a new part
tab 'Sketch' and click on '3D sketch'
make a sketch you can draw in 3D.
if you don't have the tab 'weldments'. go to a tab, and click right mouse button and click on 'weldments'.
go to the tap 'weldments' and click on 'structual member'
now you can choos a the type of profile.
click the line's you want in the first group.
click on new group.
click the next line's you want.
and doing the same witch the other line's,
first new group and then the line's.
If you're done you can click on 'OK'
this is the rendering from the frame