My sister gave me a cheap notebook from a drugstore rack the week I moved apartments, mostly because she'd grabbed two by mistake and didn't want to bother returning one. It sat on a box for a few days before I actually opened it, and it's still the one I write in every night, spine cracked now in a way the second one never got the chance to be.
Your first day
Grab any pen and paper, or a notes app, and set your phone's timer for five minutes. Write about today, starting with whatever's first in your mind, even if that's just "I don't know what to write." Keep the pen moving the entire time. Don't go back and read any of it until the timer ends.
Most people stop mid-entry to cross out or fix a sentence that came out wrong. Don't. That single habit is what turns journaling into an assignment instead of a release, and it's the main reason people quit inside the first week.
The thing most people miss
I assumed journaling was supposed to produce something worth rereading later, maybe even something I'd want to show someone eventually. What actually changed my understanding was noticing that the entries I felt best about afterward were consistently the messiest, least edited ones, the ones I'd genuinely never let anyone see.
Once I stopped trying to write well during those five minutes, and stopped treating grammar and legibility as things that mattered here at all, the notebook stopped feeling like one more thing to get right.
A misconception worth correcting
A journal entry doesn't need to be legible to someone else, or even to you a year from now, to have done its job. Fixing a sentence mid-entry doesn't make the entry better, it just interrupts the one thing the five minutes was actually for.
Nine of the last ten evenings had an entry in them before I ever sat down to count, more consistency than I'd managed with anything else I'd tried to turn into a habit. A single question written at the top of the page, instead of leaving it blank, is worth trying once the blank page stops feeling like enough on its own.