Hey from Philadelphia, I'm about to graduate from HS and the first thing I wanna do after that is to get my PPL, any recommendations or advices?
Obtain your medical certificate first. Search this forum about answering the questions on form 8500-8 especially Question 18 before you go to the Aviation Medical Examiner to make sure you won't have any surprises that could foul the process. One of which as a kid of the new millennia is being improperly labeled ADD/ADHD.
We suggest getting the medical hurdle done early so money isn't wasted on training just to find out something brings it to a complete halt or a long delay.
Do you know any good and cheap flight school in the Philadelphia/NJ area?
Being in Texas, I'll defer to folks closer to you
Is it better to take lessons with a flight school or personally with an instructor?.
Either is good. But the key with either is finding the right instructor who's teaching method fits your learning style. Finding the right instructor has been discussed many times here, so try the search function for past discussions. Also check out
http://www.aopa.org/Pilot-Resources/Learn-to-Fly
What's the next step after the PPL? Can you give an estimate of how much is the cost for the PPL?
Typically the next step is enjoying your newly earned privileges and flying to places you have yet to land at. Then when you're ready, your next training step is "Instrument Airplane" where you learn to operate the aircraft by reference to the flight instruments. This gives you the privileges of flying within the clouds without reference to outside cues such as the visible horizon.
A frequent PPL training budget we discuss is $8000-$9000 depending on locale, type of aircraft used, and how frequently you train. airplane and instructor rental is about $6000-7500 of this, and the remainder is supplies, books, testing fees, silk scarf, nomex flight suit, helmet emblazoned with either a Maverick or Iceman or Ted Striker call sign, and a few other odds and ends.
Many ways abound to keep your training budget under control and to a minimum. And this has been often discussed here, so once again, the Search function will aid you here.
@CC268 just started his journey within the last 4-5 months and asked many of the same questions you have. So look for threads started by him.
A question back to you that might help us answer future questions: Are you looking to do this for recreational flying or are you thinking of flying as a career?