How do I know when to put an 's' or not with tenses like 'je mangerai(s)'?

The difference between both is that there is one which is the conditional and one is the future. With an 's', it's the conditional which demonstrates a wish, a possible action in the future but we are not 100% sure it will be happening. The future, so without 's', is to anticipate an action that will happen in the future for sure.