If I cast a targeted removal spell like Path to Exile and all creatures have hexproof I actually wouldn't be able to cast it in the first place because all creatures have hexproof and therefore cannot be targets right? Or can I still target them and it just doesn't do anything.
No you can't cast the spell if there is no valid target for it.
601.2. To cast a spell is to take it from the zone it’s in (usually the hand), put it on the stack, and pay its costs, so that it will eventually resolve and have its effect. Casting a spell follows the steps listed below, in order. If, at any point during the casting of a spell, a player is unable to comply with any of the steps listed below, the spell was cast illegally; the game returns to the moment before that spell started to be cast.
601.2c The player announces his or her choice of an appropriate player, object, or zone for each target the spell requires.
Simone's answer is correct that you cannot cast a spell if you do not fulfill all legal targets at casting time. However, this might not be the case in the situation you are describing.
all creatures have hexproof and therefore cannot be targets right?
No, you can still target hexproof creatures you control. This means that you can legally cast Path to Exile on your own creatures if you really need that basic land.