Take the MCAT when you feel ready to take it. Some take it after their Sophomore year (having taken all the pre-reqs), but others take it long after they graduate college! Do not feel pressured to take it if you are not ready, as it is not worth it. Although retakes are incredibly common, a single strong score is the best. From personal experience, I took the MCAT twice and am doing fine this application cycle.
There are many many resources on the internet about what's on the MCAT.