Im writing a lot of metal lyrics, but i find it hard tomake the song build up, and finding themes and stuff like that. So does anyone have some tips for finding themes, etc. Thanks
Record the instruments first, then think of a topic. But always keep to the topic and subgenre when writing lyrics.
1. Acquire and read old used graduate-level pathology textbooks.
2. Watch/read old scifi classics such as The Twilight Zone and The Martian Chronicles.
3. Watch/read old fantasy/mythology classics such as Tolkien's works and the Eddur.

That should provide all of the inspiration you need.
There's no such thing; there never was. Where I am going you cannot follow me now.
Lots of metal is about touchy subjects that are more overlooked by other rock(like politics, corruption, and other negative things). Typically writing the music first would make it easier.
Music first, lyrics 2nd.

If you tell the genre, It will be easier to help. But those three themes are general for metal.