The booking form moves fastest when you have the key details ready before you start rather than looking them up partway through: your exact pickup and drop-off addresses with postcodes, the date and time, how many passengers and bags, and your flight or train number if the journey connects to one. Having those to hand means the quote you get back is accurate the first time, rather than needing to be revised once more detail comes in.
A few extra details are worth adding even though they are optional: requesting child seats, meet and greet, or a specific vehicle class at the point of booking means they are confirmed alongside your fare rather than arranged separately afterwards. If you are booking on behalf of someone else — a colleague, a family member, a guest — adding their contact details at the same time means the driver can reach the actual passenger directly on the day, not just whoever made the booking.
Once you confirm, your driver's name, number and vehicle details arrive ahead of pickup, so you have a direct point of contact if anything changes between booking and travel.