What do I need to study geology?

There are myriad types of geology, enough variety to suit most anyone. Check out a partial list:

http://en.wikipedia.org/wiki/Geology#Fields_or_related_disciplines

As for what you should study, most universities will offer a geology or earth sciences degree. In the course of getting that degree, you will be required to take a number of related subjects. I did a very quick search for the requirements for a geology degree at NYU Buffalo and came up with this for the general requirements for a Bachelor of Science degree:

Required Courses (54 cr) GLY 101-102 Global Environmental Science or GLY103-104 Evolution of the Earth and Solar System GLY 106 Geological Mapping Techniques GLY 215-216 Soft Rock: Sedimentology/Paleontology/Stratigraphy GLY 305-306 Mineralogy/Petrology GLY 312 –313 Surface Processes and Hydrology GLY 325 –326 Structure/Geophysics/Tectonics GLY 407 - Geological Field Training (To attend the UB summer field camp students must have a 2.0 average in geology courses.) Required Electives (6 cr) An additional six credit hours of 400 level geology courses (not GLY 493 or GLY 499) Required Courses Outside Geology (28 cr) CHE 101 – 102 General Chemistry MTH 121 – 122 Survey of Calculus and its Applications I & II PHY 101 – 102 College Physics I & II PHY 151 – 152 College Physics Lab I & II