Herm is the smallest publicly accessible inhabited island of the Channel Island and can easily be reached from Guernsey by a regular ferry service.
Travel Trident run the ferry service to Herm. During the summer months this typically runs hourly from around 10am, often with an earlier boat and sometimes an evening boat. During the winter months there are typically two to three crossings a day with service frequency being higher than this in spring and autumn. For details see the schedule on the website. At the time of writing a crossing costs £11 although there is a discount if you travel on the first (early) sailing from Guernsey.
The island is small so there is no public transport and only a small number of vehicles are on the island. Walking is the best way to get around.