Getting started journaling isn't something that you need to think about too hard. Yes, there are numerous types and styles of journals and ways to do this that may or may not be more effective depending on your goals, but you can simply get some paper (or your computer) and get started today.
You don't need anything special to keep a journal; in fact, purists believe that using pen and paper is the best way to journal because you can carry it with you anywhere and you don't need technology. So, there will be no excuses.
Don't procrastinate about keeping your journal. It's best to do it in the morning before you begin your day so that you have the right frame of mind for the day. Plus, you only need five to ten minutes, so it's not that big of a deal.
Another time to do it is before bed. This works especially well for gratitude journals. That way you can go to sleep thinking about all the things you are grateful for instead of things you're worried about.
Whenever you choose to do it, try to set it up so that it becomes a ritual and a habit. Journaling every single day is going to be more effective than just doing it when you feel like it.
Don't start being worried about style and substance right now; just work on the daily habit with pen and paper (or if it's easier for you, a computer or smartphone). Don't make it hard - just get going.
Start right now and write about your day today. That's the easiest thing to do. What of significance happened today? How did you feel about it? What would you do differently? What would you do the same?
Once you develop the habit, you can start trying different types of journaling like a bullet journal, or a vision journal, or maybe even a project journal for your next project.
The main thing to remember about your journal is that it should be kept private. The only exception is if you want to share thoughts with a therapist, counsellor, or coach. Or if you want to turn it into a book or course, to help someone else overcome whatever you overcame.
Keeping a journal will help you deal with the things that happen to you as well as the things that have not happened to you. The main reason is that writing it down helps you remember what you did right and what you did wrong. It helps you improve your decision-making capacity for similar situations. The main thing is just to get started journaling in any way that works for you.