Optimize MySQL database and queries

We have a web site with a MySQL database with some large tables, and other tables that will become large. Some pages are loading slowly because the queries and possibly the data structure are not as optimal as they could be. We're looking for someone who can go over our data structure with an eye toward the combinations of data we want to pull, and look for any potential trouble spots, as well as look at particular trouble spots where we already see slow page loading. The database consists of 27 tables, 18 of which have the potential to become large. There are probably 6-8 small and large tables that are called the most, often in joins with each other. There are three currently known trouble spots. Further details of the relations between tables and the potential queries used will be provided.