Sensex is dropping as a first time investor is it sensble of me to buy when they are selling??

Which industry should i concentrate on if i am investing????? Gas??? Power????

You have mentioned you are "first time investor". Yet the basic point in investment is correctly judged by you, that is sensex is dropping, our market is yet not matured, FII are taking advantage of it. One should always think of buying when prices are coming down. But no one knows how much it will fall. So the best thing is to study first, so that you will not be at loss.

Secondly which shares one should purchase. Sensex based
(30 shares) or Nifty based shares or mid cap or small cap all these questions you must consider.

Some of the good shares like Sesa Goa, Larsen and Toubro,
Sterlite Industries, State Bank of India, Hindalco, ITC, NTPC,
PETRONET LNG etc. are good for long term investment.
Power, gas, energy, infrastructure sector are better for investment. But refer to price chart and then only start buying. Because the market will remain volatile and you will get chance to purchase these shares at low rate. Don't make hurry. Long term investor with periodical monitoring never loose in stock market if keep patience in buying.

I tell you a story here: There was a trader. He thought of making money and started purchasing monkeyies at the rate of Rs.100 per monkey. People started buying monkeyis, gave the price. Then he increased the price to Rs.120. People thought that price is rising so they started collecting at Rs.120. Then he increased the price to 140. People again started more buying. By the time he collected lot of monkeyins. Now he started to sell most of the monkeyies and made huge profit. Our market is like this.

Are you referring to India's Sensex Index or a U.S. stock?
Not sure exactly..

It is always advisable to buy when the market is falling. Its considered to be an opportunity in fact.

Since you are first time investor, just be cautious. Do not over trade and do not take over positions if you do not know about the scrip. Keep strict stop loss.

Considering the scenario, shares of Infrastructure, Power and Capital goods sector are supposed to perform well in near future.