How to adjust action and into

I own a squier sonic stratocaster and I haven’t been able to adjust action and intonation by following youtube tutorials. I’m a beginner and I don’t have money to take it to the professional to do it. Can someone please explain to me in detailhow to do it, step by step. I also bought some tools such as measuring tool for action, capo and some cleaning and maintenance products that I read were needed for proper care. I don’t have a big ruler that is used for checking the curvature of the neck. My action is decent but I get a lot of fret buzz and for example when I fret the string on the 7th fret, the string is touching and playing the 8th of 9th fret. This happens on 4th, 5th and 6th string.