Jesus told His disciples that after they received the Holy Spirit they would be witnesses to Him (Acts 1:8). In other words, they would tells others by their words and actions of the reality of Jesus Christ. As Peter said in Acts 4:20, 'For we cannot but speak the things which we have seen and heard.' A witness tells others of what he has seen and heard.
If you have experienced the forgiveness of sins by being baptized in the name of Jesus Christ and if you have received the gift of the Holy Spirit with the supernatural evidence of speaking in other tongues, then you can be a witness of the reality of Jesus Christ. Just tell others what Jesus has done for you. If He has healed you too, then tell it. If He has provided for you in a dire situation, then tell it. A witness is one who tells others.
In addition to telling, we can also show others the reality of Jesus Christ by living our lives as a godly witness to the world. In other words, if you have experienced Jesus, show it by your good conduct and godly lifestyle.