I have got 420 marks Iam an st candidate in ap can i get a good govt college

Dear Poorna, AP State Government Medical Colleges are categorised into SVU, AU and OU areas. You need to be a local for that area to claim its local quota benefits. However, the cut offs for various areas for ST category are:

SVU: 395
AU:   378
OU:   421

Having said that,

360 marks got a seat in RGIMS, Ongole
369 marks got a seat in ACSR Government Medical College, Nellore

Cut off changes every year and is dependant on various factors like no.of seats available, no.of candidates apearing for the exam, top NEET score and difficulty of the exam. Considering all these factors, you have pretty good chances at getting seat in either SVU or AU areas.