The interior of the building is very dramatic. When you first enter, most of the building appears to be open space. You can see a staircase straight ahead, but not much else -- just a huge atrium that rises many stories.
But the building appearing to be empty is an illusion. There are galleries -- they're just hidden away on the sides, not visible until you reach a stairway landing.
At the very top there is a pedestrian bridge that hangs in the air and crosses the space. The floor of the bridge is translucent, so as you walk across, you can look down and see all the way straight down to the lobby, far, far below. This is thrilling and scary like an amusement park ride! Don't miss it. (You can always keep your eyes glued straight ahead if looking down gets to be too much -- I've had to do that sometimes!)
The museum has permanent collections in photography, painting, sculpture, and design, but the special exhibits are often the most interesting. There's usually a lot to see there, probably more than you will be able to see in a single visit, unless you have a lot of stamina, so I'd advise picking what you want to see most and starting there.
Tip: If you don't want to walk up a lot of stairs, take the elevator to the top, start with the exhibits there, and then work your way down. Either way, whether you walk both ways on the staircase or just one, take a look around you when you get to the mid-floor landings -- you get a different view of the building at different heights.
There's a cafe in the front, next to the entrance. I've only eaten there once, but what I had (which, if I recall correctly, was a daily special) was wonderful, though a bit pricey.