The first-time international traveller to Delhi, unless they have a driver waiting, will nearly always makes the rookie mistake of assuming that they can just walk out the arrivals gate and find a taxi rank. They assume at the taxi rank it will be just like Dublin airport, or Budapest airport, or Mexico City airport where someone is on hand to wave a taxi in and kindly ask you where you want to go and help you put your luggage in the boot and send you on your way with a clean and sober driver.
The reality is there is no taxi rank. If you do pass the money exchange and walk into the arrivals hall, you are landed in India with a couple of men clamouring for your attention asking you "Which hotel?" "Hello, how are you?" "First time in India?" (that last one is Hindi for "Just how gullible are you?"). Their friendliness is merely a sales pitch for a very expensive taxi journey. Don't assume that just because it is expensive you will be driven in a nice car.. you won't. And don't assume because you have told the driver which hotel you are staying in, that you will be brought there. They may decide other hotels pay a better commission and bring you there instead, insisting your hotel is closed or burnt down or was merely a figment of your imagination.
If, on the other hand, you pick up your luggage after getting through immigration and think "Ok, I'll keep my eyes peeled for some sort of tourist information desk or taxi booking desk" you'll spot the taxi booking desk just beyond the customs check. Delhi airport is actually very open plan, you can see the luggage belts from the immigration queue, and can see the arrivals hall from the luggage belts.
At the taxi booking desk you'll find one or two women who make the bookings, and a dozen or so various hangers on and taxi drivers. Once you indicate your desire to book a taxi, one or two of these hangers on (you could call them porters in that they carry your luggage but that really would give you a mental image of someone much neater, tidier, and trustworthy) will take your luggage from you. You won't see your luggage again until it's in the boot of the taxi and the porter is standing there explaining how the exchange rate is 50 rupees to one dollar and therefore since one dollar is such a small amount you should give them one dollar of a tip. There are very poor people in India who live on less than one dollar a day. That should give you some indication of how much of a tip 50 rupees is when you think "How much money would someone poor in
When you have stated where you want to go and paid the requested amount to one of the women in the taxi booth, she will give you a slip of paper which you are supposed to give to the taxi driver once you reach your destination so that he can prove that he didn't kill you before bringing you to where you wanted to go. Funnily enough, one Delhi airport taxi driver did actually kill an Australian woman about a year ago. He was out of his head on drugs at the time apparently, and the booking system allowed the police to capture him fairly quickly. This goes back to what I said above, you expect a clean and sober driver. Delhi airport taxi drivers are usually neither of these things.
The good news is that there are apparently plans to extend the Delhi metro to the airport. That would be great for both Delhi citizens and local and foreign travellers by taking the monopoly out of the hands of the taxi interests.