I think the biggest tip I could give is to always be working on something. Completely immerse yourself in whatever you're doing. If you think of something, write it down. Creativity wont come when you sit down and stare at a piece of paper and wish, it's spontaneous. Wherever you are, be it your desk pencil in hand, or in the car on your way to work, when that moment hits you have to milk it for all its worth. Even if its just scattered thoughts, that's better than nothing. Just looking at a few words from the past can spark a thought enough to coddle into a master piece. The hardest part is always getting started, so start. You may be shitty at first but that's more than 90% of the population can say. Even the greats where beginners at one point.

On a side note. If you're looking to produce your own music as well as write it (which you probably will at first) you should download a DAW. While lyrics can be written pretty much anywhere recording is a bit more difficult. You can always use your phone but if you have a computer I would highly recommend downloading Reaper or Audacity. Audacity is pretty limited but it is free. Reaper is a full-fledged DAW capable of pretty much anything you can do in a studio. While Reaper isn't technically "free" the license is super cheap for a DAW ($60?) and I don't believe you even need a licence to use it.