What are all the basic things that is necessary to be aware of before starting to learn geometry?
The two areas of school-level geometry I suppose are Plane Geometry and Coordinate Geometry.
Before going into Plane Geometry you should be comfortable with basic algebra. That is, manipulation, factoring/expanding, solving linear, rational, radical and quadratic functions (at least) etc. Pythagoras Theorm and formulae for basic 2D shapes is pretty well assumed knowledge also. Coordinate Geometry, depending on what level and depth you go into it, would require a solid grasp on basic and intermediate algebra, as well as basic plane geometry and trigonometry. You should also be familiar with the concept of a function.