The IFLA (International Federation of Library Associations) defines Collection Development as the "ongoing assessments of the information needs of library clientele, using statistics, analysis, and demographic projections." This includes researching new and popular books and authors, reading reviews, listening to the needs of your patrons, planning for new collections, replacing lost or damaged materials, and weeding the collection.
One of the most important parts of collection development is your library's Collection Development policy. Examples and a template can be found here. When getting started, it's important to keep in mind that the policy and collection should serve the mission and vision of the library. The library's policy should include provisions for the selection and purchase of materials (as well as any limitations), types of materials purchased, the purpose of the collection, what the library does with gifts and donations, how de-selection is handled, as well as the adoption of the Library Bill of Rights and Intellectual Freedom. These are all in line with Standard D18 of the Standards for Public Libraries, "Library board has adopted collection development policies, such as intellectual freedom, gifts and donations, and weeding."
Hone in your Collection Development skills by taking a class or reading more about it at one of these links: