You can't write from the clients browser to the server directly. So this needs server processing. The details depend on the language used in the server program; could be Java or .Net or PHP for example.
Commonly you have a database (in MySQL or MS SQL Server, for example) to write the data to from your Java, .Net or PHP program.
That's for a starter.
Hi. Grateful for some help. I want to include a form on a web site for clients to fill in their details. Then submit this information to a specific file on the web site host server, not by email. Anybody know if this can be done and if so how?