Let me first point out that there is a difference between 'having the Holy Spirit' and 'the baptism in the Holy Spirit'. Every born-again believer has the Holy Spirit of God living inside of him or her. {St. John 3:5-7; Romans 8:9; Ephesians 1:13}. This can be seen in St. John 20:22 when the disciples had the initial new covenant experience of the regenerating presence of the Holy Spirit and the impartation of new life from the risen Saviour. This happened before the 'day of Pentecost' experience.
Baptism, which means 'to immerse', usually involves three things; the person being baptized {the born-again christian}, the baptizer {Jesus}, and that which one is being baptized in {the Holy Spirit}. A simple way to explain the difference between 'having the Holy Spirit' and 'the baptism in the Holy Spirit ' is to use the example of a jug of water and a swimming pool. Receiving the Holy Spirit at conversion is like drinking a jug of water. The baptism in the Spirit is like being immersed in a pool which is full of water.
According to the post-Pentecost account in the Bible the only criterion for being baptized in the Holy Spirit is faith. In Act 2:1-4 the disciples waited in the upper room and were baptized in the Holy Spirit. The Samaritan Christians did not wait in any one place, but received the baptizm in the Holy Spirit by the laying on of hands {Acts 8:14-17}. Something similar happened to the Ephesian Christians in Acts 19:1-7. The Christians in Caesarea did not have hands placed on them in order to receive the baptism because as Peter preached they were baptized in the Holy Spirit {Acts10:44-48}.
Doubt is the main reason why many Christians are not yet baptized in the Holy Spirit. Some have carried the doubt for years because they were told that the Holy Spirit Baptism was only for the first century believers. Others do not see the necessity of the Baptism or don't think they are worthy of it.
However, in Acts 1:8 Jesus said, "You will receive power when the Holy Spirit comes on you, and you will be my witnesses in Jerusalem, and in all Judea and Samaria, and to the ends of the earth". The reason for the baptism in the Holy Spirit is to receive power to live Christ-like in this present age. Just like the 1st century believers we need power to show the world that Jesus is still alive.
Our Lord Jesus makes it clear that all we have to do to receive the Holy Spirit Baptism is to ask in faith and be persistent {Matthew 7:11}. It can happen at anytime, anywhere. Whether we are at school, at church, at home, at the mall, at work or driving in our car.