The Center for Missionary Mobilization and Retention focuses on researching missionary mobilization and retention, publishing its findings and training local church leaders and missionaries on topics which impact missionary call and missionary care. Their website contains several pages of resources, some produced by them with links to many other resources as well. Their host school, Trinity Bible College and Graduate School offers trainings for missions agencies and pastors and offers free continuing education for missionaries, allowing them to audit a graduate school class free of charge.
Ask a Missionary is a question and answer website with a range of topics: guidance on calling, funding, selecting a mission agency, families on the field, training, and professional skills. The website also has articles and book recommendations on the same topics.
Syzygy, a British missions support organization, has a helpful page of questions and answers about becoming a missionary to get people started.
Velvet Ashes, a website designed to be an encouragement to women serving overseas, has a Leaving Home series which provides women planning an international move with weekly emails offering encouragement and advice, resources to help in the transition, and a community in the form of a Facebook group with others going through transition.
For individuals interested in ministering in Israel, the Shepherd's Light, a ministry of Calvary Chapel Tel Aviv, has on their website a series of videos which help build an understanding of the nation of Israel and how to minister there.