Learn English – getting ready to go or getting theself ready to go

Yes, we still use it, and yes, it is usually a piece of cloth or fabric, typically with a specific use. As you see from your own examples, we use it more often as the plural of a compound word such as washcloth, dishcloth, or tablecloth.

For an example of the use of the word on its own, google "burp cloths". You will find all sorts of them for sale; burp cloths are cloths (pieces of cloth) that you can put on your shoulder while you're burping your baby, so he won't spit up on your clothes.

You can also use "cloths" to mean different kinds of cloth, as in "We have several cloths that will fit your needs" or "our hand-tailored shirts use only the finest cotton cloths from Egypt". Saying "cloths" instead of "cloth" here implies that there is more than one type of finest cotton cloth coming from Egypt. However, in this situation, "fabrics" is probably a bit more often used nowadays.

p.s. "EN-gram" is how I pronounce Ngram.

To do something for myself means to do it for my own benefit rather than for the benefit of anybody else.

To do something by myself means to do it without the assistance or anybody else and, by implication, without using online or other aids.

So, if I spent all my weekends picking up litter in the neighbourhood for myself it's because I wanted to enjoy a litter-free neighbourhood.

If I spent those weekends picking up litter by myself it means that nobody else joined me in my litter-removal campaign. It was a single-handed effort.

Your example sentences are badly constructed. What you are trying to say is:

I had studied English for/by myself until I joined a school.

You need to use the past tense joined.

We would probably write: until I attended a school rather than joined.