These days every airlines provides a facility to book the tickets online. When you book tickets , the seats are not allotted. You have to go to Airport at least one & half hour before to the counter of airlines which gives you a boarding pass which is a seat allotment.Now almost every airlines provides facility for web check-in facility before 48 hours which means that you can yourself book the seats as per your choice and printout your boarding pass.
This saves you a lot of time and in case you have already reserved your seat, you can reach the Airport up to just 30 minutes instead of two hours,
This is how you can do.
For example purpose , seats on Indian Airlines
Step 1: Get the PNR of your tickets ready. (When you buy tickets , every ticket has unique identification called PNR .This is also true for Railway ticket reservation)
Step 2: Visit the site of airlines. In this case , you visit www.indianairlines.in/index.aspx
Step3 : When you click “Web Check In ” , you will reach the following interface
Step 4 : Fill out the details and then you will be taken to to next window where you can actually select the seat inside the plane.
Remember for security reasons, you still have to get the boarding pass stamped at the counter.
Here is the website address of other Airlines for reserving seats