What is being filled with the Holy Spirit and how can I be filled with the Holy Spirit?
We receive the Holy Spirit becoming filled with Him; and He dwelling in us in the Mystery of the Holy Chrism (Myron) "Do you not know that you are the temple of God and that the Spirit of God dwells in you?" (1 Cor 3:16). This dwelling is perpetual and everlasting; as the Lord said "The Spirit of truth, whom the world cannot receive, because it neither sees Him nor knows Him; but you know Him, for He dwells with you and will be in you" (Jn 14:17). It is not sufficient that we receive the Holy Spirit; we should also have communion with Him. He works within us and with us; and we ought to work with Him too so that He might take part in all our endeavors. The more we are in communion with the Holy Spirit, the more fruit of the Spirit will we bring forth in our lives. A person who is in communion with the Spirit is of spiritual behavior. When such person speaks, it is Godís Spirit who speaks in his utterances. When he faces problems, he solves them in a spiritual way.
In the prayers of the third hour, we pray saying "Your Holy Spirit, O Lord, Whom You sent forth upon Your holy disciples and honored apostles in the third hour, do not take away from us. O Good One, but renew Him within us." Through prayer, we can have the Holy spirit working in us, renewing our minds making us children of God worthy to hear Him saying "Come, you blessed of My Father, inherit the kingdom prepared for you from the foundation of the world" (Mt 25:34).
You can read more about this topic in "The Holy Spirit and His Work in Us" By H.H. Pope Shenouda III.