Speaking in tongues is praying in the Spirit (Holy GHost). It is for the unbeliever, and not for those that believe:
1Corinthians 14:22 "Wherefore tongues are for a sign, not to them that believe, but to them that believe not: but prophesying serveth not for them that believe not, but for them which believe."
Tongues are also used to edify the church as well:
1Corinthians 14:5 "I would that ye all spake with tongues, but rather that ye prophesied: for greater is he that prophesieth than he that speaketh with tongues, except he interpret, that the church may receive edifying."
Trust me....you'll know when you speak in tongues. God's power is awesome, and it's at that very moment, you'll feel a change like never before!