Let me get this straight, you have an existing light and want to run an exhaust fan whenever the light is turned on, right?

I realized I can't do this now I will take power from a wall plugin so I have power all the time.. so ill be going from the fan to the plugin then to my switch
Can you help me with this

If the light is close to the fan you can run wire (typically Romex type cable) from the light fixture junction box to the fan box. Check the electrical panel to determine if the light is powered by a 15 amp breaker or a 20 amp breaker. Have a friend with a cell phone stand at the light while you turn off breakers one at a time. If it is a 15 amp breaker you can use 14 gauge wire. (14-2) If it is a 20 amp breaker you have to use 12 gauge wire (12-2). If the fan is farther away, you might want to run the wire from the switch. General rule is connect the black wire to the black wire and the white wire to the white wire. There are exceptions to that rule.

Wait a minute, let's get what you want to do straight before you jump in and create a problem.

Let's start with the simple stuff like what kind of fan and where the fan will be located.

A fan in the living room is not at all the same as a fan installed in a bathroom. The electrical codes are significantly different.

From you post, You can run a cable from your existing light to a new switch box, then a cable from that switchbox to the exhaust fan, and is OK and If near impossible to run from switch location.
One thing, that switch will only turn on and off, the fan if the light is on, and may be a good thing, when you leave, the one switch turns it off and on, you have a choice for the fan.