Can I book a flight ticket online.?

Yes.

If the airline you're booking with offer pre-selection of seats you do (for a fee). Some airlines (an example would be Southwest) don't allow pre-selection of seats; they have an open seating policy. You are assigned a boarding group when you check in, then when your boarding group is called you board the plane and pick you seat (if you're not in one of the first boarding groups then you most likely wouldn't get a great seat). If the airline you're booking on offers pre-selection of seats then you should be given the option as you're purchasing your tickets, as well as prices.

Yeah you can book online with a debit card as long as it has a visa or mastercard logo on it

Absolutely. Just go to the appropriate airline's webpage or travel agent that does online bookings.

yeah, i did mine for my trip to florida on priceline.com. There are others out there too. They send you an itinerary through your email and you pick the tickets up at the airport when you show your id.

It can be used just like a credit card but it will come off your balance immediately.

Yeah, southwest airlines usually has some good prices.

yes

of course! you can do virtually anything online now...just don't get addicted to online spending; i know people who've done it and it's not good.

You can buy anything with it as long as it has money on it.