What do you know already? Have you done any math beyond high school? Will it be your first experience with theorem-proof kind of mathematics?
There is a shitton of books on planar geometry, and you should choose based on your ability.
There's a good exercise book by Prasolov on planar geometry:
http://e.math.hr/afine/planegeo.pdfIt has solutions and problems with difficulty ranging from simple to olympiad tier.
