The rules for using the word too are actually pretty straightforward if you break things down.
First, too is used to say “too.” This rule works when you want to express that you agree with someone or want to say that you have the same situation. For example, if someone says: I have a cat, you can say: I have a cat too. You should put too at the end of a sentence, after you have said what you were talking about.
Secondly, too is used to say “too much”. For example, too hot is when something is much hotter than you'd like it to be. Or too expensive is when something costs a lot more than you are willing to pay. In this case, too is usually put in front of an adjective or adverb. For example: It's too difficult. And here too suggests that something is really too much.
Also, if you want to say “too much”, you also use too. For example: There's too much noise, too many people. Here too will always come before a plural noun, and when talking about the amount of something, it will just mean too much.
On a slightly different note: too doesn't always express negativity, as it may seem. It simply indicates that there is more of something than is needed. So it won't always be something bad, for example: I have too many friends - it might even be cool.
It's easy to remember: if you need to say “too” or “too much”, you'll probably use too. The key is to remember where to put it: at the end of the sentence if you're saying “too”, and before an adjective or noun if you're saying “too”.