When you approach a web designer and say you want a new website to be built, it is highly likely that they will expect you to know how you want the site to look, function and exist online. It is important that you have a list of things you really want on your site.
These must have features could be specific to your industry (perhaps you have bookable appointments and want an interactive calendar where your clients can book time with you), they could be specific to the type of website you want (online shops will need to be able to process payments etc) or it could be a specific theme you want to use.
Whatever your needs are, it is important that you communicate them with your website designer – they are not able to read minds unfortunately! Don’t forget, they can always offer their educated opinion on whether your idea is going to be feasable, or they can give you some ideas with regard to functionality of the site.