The particle も is used to say that something is not the only one. When you want to say 'B also does A' or 'B is also A', you attach も to the noun that is being added. If the noun already has a particle like が or を, those particles are dropped and も takes their place. If the noun has a different particle (like で or に), that particle stays and も follows it.

For example, if you say 'I like cats', then add that you also like dogs, you say 'Inu mo suki desu' (I like dogs too). But if you want to say you also went to the park (in addition to somewhere else), you keep に: 'Kouen ni mo ikimashita' (I also went to the park).

A common mistake is to say 'watashi wa mo' or 'sore ga mo' - but も replaces は and が, so you just say 'watashi mo' or 'sore mo'.

Be careful: this same word も also has a different use meaning 'as many as' or 'up to', like 'juu-nin mo' (as many as ten people). That is a separate grammar point, but you will see it often.