The secret to begin improvising is to understand how chords and scales fit together. On this page you will find a tutorial about simple open pentatonic scales and the chords that go with them. Simply click on the link below:
some resources for rgt exams and beginner guitarists
These books from the RGT are intended for study with a teacher but also contain exam entry forms. I strongly recommend you do not buy them second hand if you want to take RGT grades as they may not have the forms for RGT exam entry.