Well, there's a lot of things here. Sci-fi, for instance : pick up a book / movie / theme that touched your particularly. Do not stay at the story level (it makes the most boring songs, in my opinion), but ask yourself what's the theme is, the ideas, or how you feel about it. Write down a few words about all this. Just words, do not try yet to write actual lyrics. It makes raw material you can start working on, develop, organize.

Since sitting down with your computer apparently does not work with you, and you say your bad about expressing yourself, then let the music help you. I've read in another post that you come up easily with riffs and licks. Pick one that you feel goes well with the theme you chose - or, chose the music first and pick a theme that would goes well, whatever. Play that music, humm it. Then, try to put a few words here and there ; again, do not try to put lyrics : just words that seem to go well with the mood, the rythm of the music. Let it flow, do not force it : let the music speak, let words come. Here again, you may end up with raw material that you can work on, develop and structure.

Another tip : write everyday. To not try to write a song in one shot. Always have a notebook and a pen with you. Write words, sentences, ideas, that come to your mind, that sound well, that fit with your current mood. Read your notebook from time to time : are there pieces that go well together, that form a theme ?

You previously said that you may end up with lyrics, but you find them badly written. Have you considered using a synonym / thesaurus dictionary ? Express the same thing with different words, words that you do not use in normal conversation, make unusual words associations. The most basic concepts can suddently get a flavour, a third dimension. See what I mean ? A rhyming dictionary will help you too.