There is an experience following salvation that includes being filled with the Holy Spirit. Most people thing that once you are born again and the Holy Spirit comes to live in you that there is nothing more to salvation. You pretty much have to go on living life the way it comes at you but you still get to go to heaven. On the other hand some people think that since you have the Holy Spirit living in you, there is some sort of help in your life but nothing you can control one way or the other.
There is more to salvation than heaven and the Spirit of God being in you. In addition to salvation is the baptism in the Holy Spirit. This experience that follows your salvation gives you the entire "dosage" of the Holy Spirit. See, when you become born again you only get a portion of the Holy Spirit. The Holy Spirit you get at salvation is what gives you your recreated spirit; the new you. Now after you have received Jesus into your heart, you able to receive the fullness of the Holy Spirit, that is, the baptism in the Holy Spirit. So only people who are saved and born of God's Spirit can take the next step and receive the baptism in the Holy Spirit. When you receive the infilling of the Holy Spirit you can have proof of it also. The bible says in Acts that when the people who had believe on Jesus, or the people who were born again Christians received the Holy Spirit they had spoke with tongues directly after. So we see that evidence of being filled with the Holy Spirit is speaking in tongues.
If you want to be filled with the Holy Spirit and experience the power God has made available to us, just say this out loud: "God, I see in Your Word where you made available the fullness of the Holy Spirit. I believe Your Word and it's entirety. Now I receive the Holy Spirit and I want proof of it by You allowing me to speak in other tongues. Thank You Lord!" Now take a deep breath and freely receive the gift God has made available to you! You will feel something in your stomach begin to bubble up and you need to allow it to come out of your mouth as words. You won't understand exactly what you're saying but that's ok because it's the evidence of the Holy Spirit living and abiding in you!