Before Jesus died on the cross and rose again, He promised us that the Holy Spirit would be with us--- even IN us! So, who is the Holy Spirit?
“If you love Me, keep My commandments. 16 And I will pray the Father, and He will give you another Helper, that He may abide with you forever— 17 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. 18 I will not leave you orphans; I will come to you." John 14:15-18
The Holy Spirit is God
The Holy Spirit is the third person who makes up the Trinity. He is not an "it" or a "thing", but has a personality just as God and Jesus do.
The Holy Spirit is our helper.
You can ask Him to help you make good decisions.
The Holy Spirit is the Spirit of Truth.
You can ask Him to reveal God's truth to you.
The Holy Spirit never leaves you.
You can ask Him to be present with you in good times and in bad. His presence is how we can experience God on this earth.