I would apply whenever you feel ready for it! If you have almost all of your pre-reqs done and feel you have a decent amount of experience, why not just go for it? Remember though that you'll need at least 3 LORs. At least one from a vet (though some require 2), one from a professor, and one other one (sometimes they do ask for another academic/advisor one). Now VMCAS (which is the application system you'll be using) will let you save your data from one year and use it again if you're not accepted the first time around.
It's very time consuming though... so if you feel like your application isn't strong enough, I personally feel like it's best to put all of your efforts into improving your candidacy for vet school rather than wasting it on applications. It's also rather expensive.
As for figuring out courses or schools first. I would start out with at least a vague sense of where you would like to apply because pre-reqs differ from school to school. You'll probably want to take at least the intro bio series, intro chem series, organic chem series, physics series, 2 semesters of english, genetics, and biochem. Those you'll most likely need. But if you knock all of those out before senior year, you could leave senior year to take all the extra courses that your schools would like you to have. (Or I guess if you're going to apply junior year, have the core courses done by sophomore year and leave junior year or at least the last semester to knock out the rest).
Of course, all of that confusion can be avoided if you pick at least a few schools that you KNOW you're going to apply to. Then you can just add more schools later that require the same pre-reqs you've already done (or maybe just one or two more that you're willing to take).
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