How should I buy this Ticket online.?

You can only buy tickets on line if you have a credit card. If you visit your local travel agency or call the particular airline direct they will usually be able to match or sometimes beat the price you've found on the internet. An airline or a travel agency will be able to accept your bank's card whereas the internet will not - (unless you know someone with a credit card and they pay for the ticket and you pay them back.) The only times travel agents don't match the price is if you are booking a hotel on line - but since you are booking flights you will be OK.