Hello,
I'd recommend 'Learning Teaching' by Jim Scrivener. This is one which is recommended during CELTA courses and is a nice introduction.
There is also a series of books called 'How to teach ..'. For example, I liked 'How to teach Grammar' by Scott Thornbury, and the series includes 'How to teach Vocabulary', 'How to teach Writing' and so on.
Decent coursebooks such as the Headway series also have quite a lot of good ideas in the teachers' books, which can be helpful when you're starting.
I do recommend a CELTA though if you can manage it in the future, it's really useful to get such a lot of feedback on your teaching.
If you need any help with your grammar (if you are a native speaker, and so perhaps not completely au fait with the ins and outs of English grammar) I'd recommend 'Grammar for English language Teachers' by Martin Parott, and 'Practical English Usage' by Michael Swan.
Best wishes,
Seonaid