I am trying to patent something. Should I write a provsional patent first or write a patent first?
You have specifically asked about writing an application, in the sense of drafting. In this case, I would suggest there should be no difference between the two. While provisional applications have somewhat lower formality requirements, you would generally be best placed to draft it as if it was a non-provisional. This is because if you file a rough sketch on a napkin as your provisional, your redrafted non-provisional that you file 1 year later will likely differ so much that the provisional will not give you any real priority claim. It would be as if you never filed the provisional.
If you meant to ask about filing, then there is little downside to filing a provisional patent application first. This is the cheapest option to ensure you get a full 21 year term (the 20 years of a patent plus the 1 year of priority). You could achieve this with a non-provisional patent application, but the filing fees are slightly more expensive.