Hi,
I don't know a good introduction to using WL/Mathematica for solving differential equations for people who are new to Mathematica. There are some good introductions to Mathematica, but differential equations is usually a later chapter, if it appears at all. For instance, this nice introduction does not cover it: https://www.wolfram.com/language/elementary-introduction/3rd-ed/
The documentation bundled with Mathematica has this tutorial/book Symbolic Difcerential Equation Solving Chapter 1 and the beginning of chapter 7 might be of use to you). There is also Fast Introduction for Math Students, which has a chapter on differential equations. The problem with most Wolfram documentation and books, imo, is that they do not show new users how to find and correct the mistakes that new users commonly make. A troubleshooting guide is needed here, but I do not know of one.
Sometimes the documentation for the error message has troubleshooting info, but often not. For some years now, Wolfram has hidden the message name in the popup that appears when you click on the three red dots next to the warning/error message. The popup menu has a link to the documentation for the message, and that might have information that helps you debug the problem.
The message name is a key to the problem that might help unlock it. I don't know why it is hidden like an irrelevant technical detail. On Stackexchange, when I come across a question about a message, I add the name if it's not there, so that the issue may be Googled. A message is a symptom, and the same symptom can have several causes. Each Q&A on Stackexchange tends to have a single cause, and so you may have to go through several hits to find a fix to your own issue. Again a troubleshooting guide that goes through several possible causes for each common message would be helpful.
Google Set::write
on Mma.SE
Google DSolve::dsfun
Mma.SE