To get a ticket to enter the Acropolis and head up to the top, you have to pay 20 Euro at the ticket booth. However, there's two lines to buy tickets. One line is for credit cards and the other one's for cash. There's little or no signage to know which line is which, so make sure you're in the right one. This leads to my next tip.
5. Eat before you go and pack lots of water.
The ticket lines are long and you'll want to spend several hours exploring, so make sure you go on a full belly, pack a snack, and carry water with you to stay hydrated in the lines and up on Acropolis. There's lots of cheap gyro places with bottles of water near the entrance to the Acropolis to get stocked up.
6. Prepare for the intense sun and bright surroundings!
This means pack your sunglasses, hat, I recommend an umbrella even, and sunscreen. The Acropolis and the area surrounding the Parthenon is very exposed to the intense sun and has little shade. To make the sun's rays even more intense, there's white rock reflecting light at every angle making it super bright up there. So, make sure you're ready for it. Check out my packing list for visiting the Acropolis and Parthenon here.
7. Wear shoes with good traction.
The ground is uneven and it's slippery up there! The rock is so warn down from centuries of heavy traffic, I was slipping around up there with my shoes with no traction. I don't recommend flip flops. Make sure to wear shoes with good traction. Again, see my packing advice here.
8. Take your time and enjoy it up there!