How do i just sit down and write?

ive had an idea brewing in my head for a novel for the past 3 years, but i never wrote it down because im lazy/i lack confidence/i think no one will take me seriously. now, i think i should at least get the story on paper.

i always get distracted with a computer, so thats why ill write my first draft by hand. but theres still the problem of my add. im on meds, but i cant still cant focus as much as i want.

should i just take 120 hours straight, with coffee, caffeine pills and take out? i heard the best solution for people with add is to just “get it over with”. or should i try a different approach. any tips would be nice.