If you find it hard to find things to write about, which sometimes I do, I write a sentence or two for each day of the week (as seen below). Other things I regularly do is write in the outfit I was wearing that day (or on one of the days where I thought my outfit was particularly nice!) or write in the music currently playing in the background, current mood or the current weather.
If you're really stuck though, I made some prompts for a listing challenge last year but you can also use them as journaling prompts. Alternatively, Life Captured Inc is a good resource for prompts too.
I think it helps that my journal is so cute and pretty so I would definitely advise investing a journal you'll love to dig out time and time again for at least six months (or longer, depending on how long it'll take you to fill it). My aim is to keep journaling at least once a week this year and fill up this lovely Anthropologie journal :)