I use NotepadWithPencil Version A4 and Brain Version 1.0 (I believe) to start. Sometimes MS Visio is quite handy, specially if you have an existing database and you want to get an overview about existing tables and relations (not always works). If it really needs to be professional I prefer PowerDesigner. Check this link: http://www.sybase.com/products/modelingdevelopment/powerdesigner