The sooner you make this decision the easier it will be to plan. Most students go in their junior year, but there is no reason why you cannot go sooner or later. It takes time, though, so give yourself a semester at least to get it set up. There will be a need to coordinate your major and the program you will go through. So when you have decided that you want to go for it, choose a semester or year to target for this experience.
Nothing is in stone yet, though.
Throughout this I shall use examples from my experience. It is because I know how scary and complicated it seems to go through all this, that I am writing this. I went through SUNY New Paltz to Oviedo, Spain in the Spring of 1997. This was a program in a different school from my college, because I was a Le Moyne College student. This is very common to go through other college's study abroad programs.