Contact the venue to see if the date you want is available and find out what packages they have to offer. Pay attention to any details they insist on – minimum numbers of attendees, menu options, corkage charges, any limitations on entertainments, timings, logistics or decoration, the deposit they require to secure the booking, whether you have exclusive use of the room(s), etc.
You may want to investigate a number of potential venues to check how each compare and which offers the best deal.