Hi,
I'm about 2 months into my phd and i have kept a 'journal' from day 1 as I did this for my masters and found it really useful. mine is basically just a plain notebook, and every morning i write down what i want to achieve that day, e.g. read a certain paper, complete an experiment etc. then during the day i add pretty much anything that pops into my head, such as ideas, other things i need to do, emails i need to send... then at the end of the day i go through it and check off tasks i have completed.
sounds pretty time-consuming but once you're into the habit its pretty easy, you don't have to write much and it can even be just a few words jotted down to remind you of something. i find it much better to have all my scribbles in one place rather than on loads of random bits of paper!
i also use it to write down anything that hasn't gone so well (or things that have!), things that annoyed me etc as i find writing these things down really helps.