Hi ya,
The best place would be to fly into Monastir airport and then you can get a taxi to Port el Kantaoui - this should cost no more than 15 dinar so make sure you agree a price before as they can be cheeky and put the cost of the taxi up. If you have to fly into Tunis airport then your best option would be to book a coach transfer as its about 2 hours to Port el Kantaoui from Tunis.
When you are on the airplane, they will give you a little visa card to fill in your details and then hand this in at the passport control they will take the section they need, this is your valid entry and you can stay in Tunisia for 3 months maximum (I'm guessing your not staying that long but just in case

).