I need help. . . I can always make stuff up on guitar but I just don't know how to start writing lyrics. Any tips? Where do I start?
think about the mood of the song. is it agressive, party, major/minor key, then pick something to write about that fits the mood. at least that's what i do. and yes, it is most definitely okay NOT to rhyme.
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Whenever you come up with lyrics make sure you already have some riffs or a song to go along with it. IMO it's just easier to make the music, and then the lyrics after.
451F is completly right about the mood of the song, and also this might sound weird but using stuff you learn in school always works for me - If its about a war, or some myth (Flight of Icarus by Iron Maiden demonstrates this) or something like ghengis khan its always good to have tons of knowledge about what your writing about. Tom Morello (RATM) writes his lyrics about politics, which he studied throughout his life in school.
what I do is come up with a hook which is like a line or two and just build onto it.

do whatever you want the lyrics don't have to make sense just listen to I am the walrus by the beatles... random phrase that make no sense yet people love it.
it's sometimes easier to find a melody before you write lyrics as well, so you can fit your words into the melody.

try to find a theme, be it war, politics, love/relationships, life, whatever.
It is a difficult thing to do. Something I'm still struggling with. Try leaving the complex parts of the riff for when you're not singing, so you can focus 100% on playing, then sing while playing the simple parts.
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This is coming from someone who's basically in the same position as you but really all you can do to start out is write SOMETHING down. I've realized that the initial block with writing lyrics is a fear of writing down something stupid, more than anything else. Just write down random words and phrases you can think of no matter how stupid they are. If you keep doing it a little bit every day, eventually you'll come up with something good.
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vartanspartan is absolutely right. right the first few lines you think of. you can always go back and edit them, but once your mind is in the flow of writing, keep going and dont stop until you start trying to think of things. go back and revise whatever you've written, make it flow, and keep building on what you've just written, and you'll eventually have a song.