depends.
Some people plan before writing anything, and some people write as they go.
Maybe figure out which way makes you the most comfortable to start writing and then just go from there. There are no strict rules, so you can also plan a little and then write and plan more while writing and vice versa.
Also, dedicate some time to actually write and try your best to minimise or remove all distractions such as your phone, messaging apps on the computer, TV, etc... Some people have a goal of 500-1000 words a day to start or maybe an hour of time a day. But also, don't rush out stories. Write at your own pace and hone your skills. Get feedback on your writing from people, but always take their feedback with a grain of salt.
Another tip I would give would be to write for yourself and not for others. Write what you enjoy writing and in the style you feel the most comfortable using when telling your story.
Once you get in the groove, you'll eventually keep improving, and viola, story done!