People are told to empty their minds, but God never tells Christians to empty their mind. They must be alert, aware, conscious of what they are doing. They were speaking in their tongues. There was no learning to speak, no going over gibberish until it came. There and then they were given the gift of tongues.
Languages take time to develop.
Most of us start with a very limited vocabulary. Gradually it develops. Tongues are like that. It takes time to develop the gift. Don't give up.