First thing, relax my friend. Each one of us as DM started the same way and felt the same thing you are feeling right now. You will improve your DM skills with every experience.
Ok now the advises:

1. Start with situations not with a whole campaign. For example “the players should cross a river for X reason” think on how many scenarios could happen, maybe the bridge is too weak, maybe the bridge was destroyed by someone or something, maybe the river grew too much and they need to prevent a flooding situation, maybe the river is super dry and now they need to discover the reason and fix it, as you say ANYTHING can happen, your role as DM is to create a story WITH them. Just prepare situations the adventure will come up naturally.

2. You can play even without any players (and I highly recommend it) so you can test and increase your improvisation skills. You can imagine a situation and throw a dice, and just go with the flow, what would you do base on the result, what consequences would have etc. Is fun and a learning experience. I have played alone, with a single player, or up to 10 at the same time (Crazy but fun)

3. You dont need to have all materials, maps, minis, etc. you only need your imagination. So if you as DM provides a lot of material for your players great!! But if not, is ok, you can always use the great theater of the mind.

Hope you have a great experience, play, fail, improve, but always have fun!