for travelling to houston from NY by air do i have to buy ticket for 18 months old daughter?

I am going to buy ticket from website , what should i mention ?

yes, you will need to buy a ticket if you go direct to the airline website, specify 1 child under the age of 2 (in seat) if the 18 month old child is in your lap it is free, if you purchase a seat you pay full price.

Check with the airlines involved.

The rule of thumb is that, if she is going to use a seat, you will have to buy a ticket. Otherwise, she will be sitting in your lap !

No, just mention child in lap.

The FAA requires anyone over age 2 have their own seat.

Those traveling under age 2 are allowed to sit on your lap. You may want to bring a birth certificate to show proof of age and make sure you advise the airline you will have a lap child.

2 other things...

1. Carry your car seat to the gate. If the flight has open seats most gate agents or flight attendants will allow you to seat your child in the car seat for no additional charge.
2. If you choose to buy your infant a ticket, many airlines offer a 50% discount for children under the age of 2.