As a general rule, when telling a story, do not change tenses. Telling a story of something that happened dictates that it should be in the past tense.

If you write something about what you will do or something that will happen, then you will use future tense. But in such writing, you may also refer to things that are happening now (use present tense) or things that happened before ( use past tense).

Don't worry too much about that, and just keep writing. On Steemit, we have a fantastic platform for writers, so take advantage of it. I believe it will grow exponentially in the coming years.

Good luck!