You can use the adverb hereafter to talk about what happens next, after this moment. For example, you might say, "I will hereafter get my homework done in time."
When you mean "from now on," you can say hereafter, like when you give a friend a new nickname: "You will hereafter be known as Brostache." Another kind of hereafter is what happens after death. When someone talks specifically about "the hereafter," he usually means heaven or life after death. This is the earliest meaning of the word, and it's often mentioned in religious contexts and at funerals.