Hello I just started playing arkham horror, and have an quetion on monsters on a portal. let's say my investigator lands on a portal and it has one monster on it. Do I still automatically get sucked in, or must I fight the monster first?

You have to deal with everything in the turn order. So:

Phase 2 - Movement: Move to the space and deal with the monster (either fight or evade)
Phase 3 - Arkham Encounters: Get sucked through gate in the Other World
Phase 4 - Outer World Encounters: Have an Outer World Encounter