how to get the tickets
There are several options how to get the tickets. It is up to you how many days you want to spend at the EXPO. Some of our group bought ticket for 2 consecutive days in advance here, we bought them on site. Fixed date tickets can be purchased in advance only. EXPO is opened from May 1 to Oct 31, 2015 and maximum capacity each day is 250 000, so I think there is no chance for tickets being sold out. trip to milan is not such an expensive thing, if you are lucky you could have 60 eur for return flight ticket + 60 eur for 2 day expo ticket + 60 eur for accomodation (4 nights shared airbnb)
how to get there
EXPO site is situated north-west of Milan and you can easily get there by metro, we took red line from Duomo to RHO Feira, you won´t get there with regular ticket which costs 1.50 with 90 minutes validity. I reccomend you to buy special return ticket for 5 eur at office at Duomo metro station to get to the expo instead of buying two for 2.50. Expo has 4 gates so better check here if you are coming by car. The daily opening hours are 10:00 to 23:00.
what to bring on site
there is security control at the entrance so no knifes, no razors or scissors. I am not sure how it is with umbrellas but i think you can take one. I recommend you to take bottle of water, there are many fountains with drinkable water where you can easily refill the bottle (they have sparkling water too)
it depends on weather, but don´t forget to use sunscreen, most of the time you are in the shadow but you never know, sunburned neck is nothing you are wishing for.