The best guide for learning asm can be found here:
http://media.taricorp.net/83pa28d/lesson/toc.html
Although when I was first learning Hotdog's asm guides were also really helpful:
https://www.omnimaga.org/files/Our-TI-PC-Products-Music-and-Calculator-Videos/Our-TI-Documentations/TI-83-ASM-For-The-Absolute-Beginner.zip
The best method to learning assembly is to learn it how one would do any language such as basic, axe, or the like is to practice practice practice. This involves also not trying to push yourself farther than is realistic when you are first learning; although this depends on the person. Also, really get used to reading A LOT. Even when you see a giant wall of text such as this one; just power through on the tutorials or code, whatever you may be reading; it will greatly help you and avoid a lot of potential questions
Also, when learning something such as asm, it is really helpful to have a nice environment set up so you can easily write programs and test them in an emulator so you can see first hand what happens, plus it will give you a good base for debugging when you go about making your own programs. SPASM is the recommended assembler for z80 code; there is also Brass but I find it to be not as intuitive for first time users.
Lastly; feel free to ask any questions you may have! Don't feel discouraged if you can't figure something out, just keep trying different things until you get the desired result. Expect a ton of failure; it is a difficult language to learn, but once you get the hang of it you will find it to be easier than Basic with many respects.
Don't give up! Just keep trying a little bit at a time; and be sure to read