I have also applied for it in lahore. And my exam is on the 25th nov. my father happens to be an army officer, and he's helping me. But i have a couple of friends who gave their initials in the previous session, so i am actually more inclined to getting tips from them! I hope this helps, basic subjects would be english, maths, pakistan studies, islamiat and general knowledge. English and maths are pretty basic, no hard stuuf there. For english they'll check you communication skill during the interview and your grasp over the subject in the mcq's. Maths, again pretty basic, basic addition, subtraction, division and multiplication. My friend said that they ask questions like whats is the half of 1/8. Now for pakistan studies, the prep is pretty easy for pk. Stds. Just get a map of pakistan, and go through every region. Provinces, city's location, rivers and stuff like that. Also, u should know the capitals of the rest of the countries, basic locations, continents and etc. now for general knowledge, g.k is all about current affairs and whats happening around you. Let it be from within pakistan or worldwide. Start reading the news paper asap, magazines, news blogs and stuff. Now for islamiat, i have no idea regarding it, sorry. 
for the interview, a couple of very basic but important tips.
- confidence. Be confident, no matter what they ask you, answer confidently. This is one of the most crucial points to get in the army.
- no lies. DO NOT LIE at any cost. No matter what happens, dont lie. Even if they ask the most emabarrassing or stupid questions.
- no 'tukka'. If they ask you a question and you dont know the answer, just say 'sorry sir, i dont know the answer to this'. Wild guesses while answering the questions leaves a bad impression.
I hope this helps in one way or the other, if u have any more questions fire away. I'll be glad to help. and best of luck. InshAllah i hope both of us get in.
P.s im worried about the medical tests only, if you know what i mean. 
