Acts 2:8: But ye shall receive power after that the Holy Ghost is come upon you: and ye shall be witnesses unto me both in Jerusalem, and in all Judaea, and in Samaria, and unto the uttermost part of the earth.
The Holy Ghost, one of the Trinity, is given to us as a comforter as well as to help us be witnesses unto Jesus throughout the earth. Until the disciples were filled with the Holy Ghost, they never went out to testify about Jesus.
As soon as they were filled with the power of the Holy Ghost, they were transformed into firebrands for God. They testified boldly for Jesus and God began performing miracles in their midst. Sinners were repenting and getting baptized and were being added to the early church. The sick were getting healed everywhere and God started to move like never before. Nobody had seen such a thing except when Jesus lived with them and performed miracles. The disciples never did a single miracle when Jesus lived with them, but when the Holy Ghost came upon them and anointed them, they were filled with great power.
The Holy Spirit anoints us with a purpose. We are to set our lives apart for God when we are filled with the Holy Spirit. If you want to be used by God and to be His witnesses, you can do it effectively through the infilling and the power of the Holy Ghost. Be filled in the Holy Ghost.