It really depends what you want your journal to be and how you like to do things, and how familiar you’re with the act of writing. So, it’s a bit difficult to suggest anything specific without knowing you a little more. What follow are general advise I would share with most people. Pick whatever you find useful while ignoring the rest ;)

Imho, it’s important to keep in mind that motivation comes and go. Whatever it is we’re doing. It’s also key to be fine with the fact that you won’t journal every single day, that’s not just fine that’s normal. I’ve been journaling for almost 50 years and I have made many pauses during that time. Some lasted a few years. We’re both fine, my journal and I ;)

As for actually writing in your journal, the best approach is to keep it simple. So it’s never feels too much an effort to open your journal. How do you do that? By not putting too pressure on yourself and by simplifying. It all boils down to that.

• Have a place where you always keep your journal (if it’s not digital, I mean) and have it easily accessible, also keep nearby the pen you like to use. Have a small space where you know you can instantly sit and start writing (a desk, a kitchen counter, whatever but that should be somewhere you know is always available). If you need to clean the space every time, or if you need to hunt for the notebook or for the pen every time you want to journal, you will probably hesitate to do it. And you will quickly give up entirely.
  My journal and my pen are always available on my desk. They stay there. No exception.
• When you’re journaling, make it short. At least, as a beginner it’s a good idea.
  People who do not write much often underestimate how taxing writing can be (physically and mentally), and how time consuming it is too. You could try, obviously, but I would advise against writing down your whole day. Write the few stuff that truly matter to you. With practice the writing muscle will strengthen and you will be able to write more and faster.
• Don’t try to write a novel or poetry and don’t try to write a philosophical or a psychological essay. No matter how talented you are, you can’t. Not every day. Nobody could. Not even Flaubert would write great sentence all the time (even though he was rather good at writing great ones more often than not he also wrote meh stuff). It would be like trying to paint a new and different Mona Lisa every single day. It won’t happen. So, be OK with writing stupid shit, or you will quickly get demotivated
• Do not obsess with writing nice sentences either.
  It’s a journal, not a book. Write down things as you think about them, as they pop into your mind, clumsily if they’re clumsy. I do mistakes and I also never hesitate to write stupid stuff or even things I know I disagree with while I’m writing them down. If they want to get out, I let them as there are no consequences. I will sometimes cross them out, but I do not worry about messing up the journal or looking dumb because of what I just wrote in my journal. That’s part of who I am, my many flaws included ;)
• Don’t be afraid to make a mess in the journal. It’s OK if some pages are not pretty. I explained somewhere I have this habit of ruining the first page of each new journal (technically, it’s the second page but that’s another story). I doodle, I make literal ink stains, whatever that’s messy and 100% braindead. Now that my nice journal is ruined, it can freely write stupid shit in it, and sketch badly.
  Did you just ruin a page? No big deal, turn the page and you’re free to start again.
• Try to add stuff to your journal to make it funnier? I like to sketch, others may like to do collage or whatever. It’s your place to experiment.
• In another post, I mentioned the first things I tend to write for each entry is some key infos—key to me I mean, they’re worthless to anyone else. I just want to be able to quickly find them if I ever read those old entries. They’re quick to write (a few ords at most but they help start writing and I think it makes easier to write more about the events of that day. Not every time, that’s fine.
• Be OK with failing and even giving up. I’ve done it for 50 years or so and I’ve ‘quit’ journaling many, many times sometimes for a few years. So, what? It’s not a competition and there is no prize to win, there is no one to beat.
• A last suggestion that may sound silly but I really don’t think it is. It’s OK to not like writing in one specific journal or using one specific pen. You can close the notebook and put it away before it’s filled, just start a new one. The same for the pen you’re using. You’re not married to either one.
  Comfort and enjoyment are essential. I can write with a ballpoint pen, but I almost never journal with one as I much prefer using a fountain pen. It just feels so much better to me. Likewise, I use a notebook whose paper feels great with a fountain pen, and with an ink I like to look at. Even though it is never a necessity, it’s perfectly OK to treat yourself ;)

Btw, it’s also a lot less tiring to write with a fountain pen or with a pencil than with a ballpoint pen. A good middle ground would be a gel pen of some sort.

Hope this helps.

Do not hesitate to journal about your impression and what you think of those suggestions if you try any. And don’t be afraid to share your conclusion with the others here :)