Having the right size comfortable boots is key to your enjoyment of the holiday, cold or sore feet will spoil your first time experience.
You will probably be renting your boots from a ski hire shop, this can be done through your tour operator who generally gets favourable prices, but it is worth checking the costs of booking direct with an equipment hire company.
There are usually queues at ski hire shops on the first day when everyone is picking up their equipment, if you can pick yours up the night before then all the better.
The hire shop will ask your shoe size so make sure you know the shoe sizes of all your party in european or US sizes. Ski boots are actually sized using a different system, but they will do the conversion. Take your time to ensure a good fit, but if you choose incorrectly you can always go back at the end of your first lesson and change them. Do this; don't suffer with an ill fitting boot.
You will not have worn ski boots before so they will feel strange. Wear only one pair of good ski socks to try them on, make sure you can move your toes or you will be cold, but they should feel snug without too much lateral movement. They will also loosen slightly as they are used and they mould more to your feet.
Familiarise yourself with how they fasten and undo, and remember to loosen the catches when you are walking any distance.
Hire boots are perfectly adequate if you take your time with the fitting, but if you are considering investing in one piece of equipment then this should be boots.
You will save money in the long run on hire and you can ensure a perfect fit. Ski shops offer custom insoles which provide an even better fit and improve performance. Go for an intermediate boot, you will soon improve.