To be filled with the Spirit means to allow God to take you over completely, spirit, soul and body, nothing is off limits to Him. You have given Him free course in your life, so that He can fill you with Himself.
This is not a onetime experience, but a continuing experience, it's like breathing, it's your very life.
Being filled with the Spirit and the fullness of the Spirit is not the same thing. Any born again believer or child of God becomes fill with Spirit or the Spirit indwells him/her.
In order to become filled with the fullness of the Spirit, we must spend time in His presence daily in order to allow Him to take control of us. I believe we obtain the fulness through trust, and in order to learn to fully trust God we must get to know Him, so that we will relinquest all to him.