They are usually used interchangeably. However, keep in mind "too" can be used to show degree...for example, I ate too much food. I ate too little food. This room is too cold, and so on. "I, too, feel that way" &" I also feel that way" have the same meaning.

They are usually used the same way. There are only differences in placement. If you want to say, 「友達も行きました。」you can say, "My friends went too." But when you use 'also' it sounds better to put it in front of the verb, "My friends also went." 'Too' is also used to modify adjectives, "This tea is too hot." You cannot use 'also' in this sentence, "This tea is also hot," is wrong.