Spring of your junior year is an excellent time to start researching your interests (i.e. for college majors/interest areas) and to reflect on what you want out of your college experience. Once you know sort-of what you'd like to study and some other particulars that you want in your college (beyond location and size), then you can start researching colleges. A great website to start this process is www.bigfuture.org, which is the College Board website. If you can go into the summer before your senior year with an evolving list of colleges you are considering applying to, then you can get a jump start working on college applications before the hectic senior year begins.
Also, spring is a great time to take an ACT or SAT test, and you can use the free resources provided by both those companies to do practice tests and test prep.
There are obviously more things you can do during the spring of the junior year to prep for the college process, but these are the bigger items to concentrate on first.
I'm happy to answer other questions either through email or you can schedule a call if you want to discuss further.