お is an honorific prefix, you use it whenever you want to be a bit more polite. Some words you can add the prefix directly like 水→お水 etc whereas some other words will already be the standard way to say them, such as お茶, the お here is already implemented in the word.
As a rule of thumb, お is generally used for kunyomi readings and ご for onyomi readings. Both have the same honorific fonction, and both are written as 御 in kanji.
The generic rule seems to be that お is applied to Japanese-origin words where kunyomi is used for the kanji that follows お, while ご is applied to Chinese-origin words where onyomi is used for the kanji that follows ご. However, there are exceptions to both of these. More (and better) explained in the tofugu article. Hope this helps.
It's generally placed before nouns! Could be written as御 attaches to nouns that are native Japanese words, the words that take お are often related to what you use in daily life food and drinks, or things related to everyday tasks: お箸；お米；お酒；お水；お茶；おつまみ；お肉；お野菜；お食事；お醤油；お醤油；お皿；お金；お車；お手紙；お買い物
Can also be used for showing respect and reffering to a family role. In this case, it's generally combined with a name ender, like さん or ちゃん. You can add お and さん to words likeお父さん；お母さん；お兄さん/お兄ちゃん, お姉さん/お姉ちゃん -Why isn't お used for弟 or 妹?
Traditionally, you don't show the same level of respect towards someone
who is younger. To politely refer to someone else's little brother or
sister, you can still add a name ender like弟, or妹ちゃん, but not お.
InContrary ご is generally attached to nouns that are of Chinese origin! These words are often abstract nouns, meaning they aren't physical objects: ご住所 ； ご注文 ； ご結婚
It's good to have all this data, but with things like this, I generally recommend mimicry - start using it as you hear it being used for certain words by other native speakers.
