So it looks like the question has been answered for the most part by others but here are my two cents.
You should start looking now. The sooner the better. You want to give yourself as many options as possible. It can get pretty hectic during junior/senior year depending on the classes you're taking and you don't want to be in senior year frantically doing last minute research into colleges. I would advise you to make a list of any colleges you want to go to or show interest towards. Do some research on them and over the course of a few months, slowly narrow down that list.
I'm going to go on a bit of a tangent but make sure when researching colleges to take into account finances, location, and educational requirements. I would argue that half the difficulty of college has to do with finances. You want to exit college with the least amount of debt as possible. If you decide to go to a college that doesn't give a big enough financial package, you might end up having to take a few jobs which in turn might hinder your performance in classes. This is even more important if you're out of state. Typically, out of state students end up paying more not to mention you have less resources to work with since your family most likely isn't going to be moving with you. Finally, make sure you know the colleges' educational requirements and try to exceed them. Admission officers have to go through thousands of applications and you want to stand out from your peers.
Kudos to you for starting to think about it now. I agree with quintavius that the best time to start is now. It can be an overwhelming process and difficult to squeeze in campus tours into your schedule but take it in little chunks and hopefully you'll actually find it to be a fun experience.
Start by doing some research. If you have an idea of what you'd like to study, definitely factor that into your research. If you're unsure, look for schools with strong liberal arts programs. Consider what area/geography you'd like to be in.
Many schools have online tours now which can give you a feel for the campus. Many universities visit high schools which will give you an opportunity to talk to the representative, get an idea of their acceptance criteria and gather some brochures.
Once you've narrowed it down a bit, you can tour the campuses in person. You'd set up your tour appointment through the admissions office. I'd recommend touring a small, medium and large sized school to get a feel for what feels right to you. You can then target your search even further based on that.
Many schools let prospective students stay overnight in a dorm room with current students - something to keep in mind if you'd be interested in that.
I hope this helps.
Best of luck to you! Remember to enjoy the process!!
I would start looking for colleges in your junior year. Also, I would consider getting your basics at a community college. This will not only save you money but will give you additional time to decide or get accepted into the college of your choice. Also, I would highly encourage you if possible to meet with current students at the colleges you are considering. This will give you true insight into the experience. I would not rely on those "college ranking publications" for guidance.
Good luck on the journey!
I would suggest that you start narrowing down your colleges list.
Baljit recommends the following next steps: