It will be freakin' hot and likely dusty, so dress accordingly and bring plenty of Murine or Systane for your eyes! Hat and sunglasses a must.

Subway line from the airport terminates at the main circle line (also an intermediate stop connecting to another eastern city line) but is considered separate with its own fare structure and no transfer to the rest of the system. IIRC it's 25Rmb versus 2Rmb for the other lines.

If you are arriving mid-to-late afternoon, you'll hit the main subway system at rush hour when it will be even more crowded than normal. If you have more than a backpack, or single roller bag, it will be hard to get through the crowds. Even with minimal luggage, be prepared for the crush. If there are few of you, it might be best to do a cab to your hostel which with three people would be marginally ($5) more than the subway combination. Otherwise, subway is easy to get around on once you know which lines go where and as noted the English version of station names are posted under the Chinese lettering. Same with major streets.