Moving to MySQL or MS SQL

Guys, I am very good at MS ACCESS database and VB. But am tired now working on them. I need to promote myself to MySQL or MS SQL so that the huge DBs I have started developing can be used by multiple users at the same time i.e. entering data in the same table by many user at the same time.

I want to develop a web based DB that can be put on LANs and WANs, please how do I start?