1. Write in something that makes you happy. That way, the first thing you can write down is your journal itself! I wrote about how a cute, but relatively inexpensive, notebook from Target is good for me. For you it might be a Moleskine notebook (rumored to be used by Hemingway himself and beloved by reporters) or a 1-subject spiral with an adorable kitten on it. You don't have to spend much money on it. Just make sure it's a dedicated notebook used only for these entries. The point I'm getting to: if your journal makes you happy, you're more likely to write in it!
2. That being said, don't put off starting until you find the "perfect" notebook. The book doesn't matter nearly as much as your words do.
3. Keep a writing instrument with it at all times. I use a pencil because I'm a grammar freak who hates to cross out when I make corrections, but a pen is fine. Hide it if you need to, if the "pen fairies" visit your house and steal all your pens, the way they do at mine. I tend to write in my journal right before bedtime or if I get a minute during the day. If I have to spend ten minutes looking for something to write with and probably getting distracted while I'm looking, the moment may be lost. If you have a pen handy, you're more likely to write in your journal!
4. Don't be overly concerned with "correctness." This journal is for YOU. It doesn't matter if you spell every word correctly (just ignore what I said up there about making corrections. I'm a freak and you shouldn't follow my example. Plus it's quick and easy for me to make quick edits.) I don't even use complete sentences most of the time (gasp!) I use a bulleted list like this:
6. Try to write in it regularly. I usually don't do it every day, but I still make it a habit to jot something down every 2 or 3 days at least. If I don't, I'm pretty sure I'll stop doing it.
7. Occasionally read back over your entries and smile!