No one else in you church speaks in tongues, either. But, if you want to pretend like other Pentecostals, just relax and start stringing together random, simple syllables from your English phonetic repertoire. "Bue la la..." Make worship about entertaining you, not about honoring God, and you'll be on your way, and cheer when your pastor preaches blasphemies.
See, you're not relaxing enough. They're showing you how to do it. Just cut loose. The uncontrollable state is almost necessary to achieve randomness, else you'll spend time thinking about what syllable to utter next, and that just won't feel authentic.
There's an opportunity for you. If you can't fake tongues, you can fake interpretation. After you hear someone make some random noises, just start spouting off some hollow language using your Christian language repertoire, "I'm the Lord your God and I say to you, <insert a belief particular to your denomination>...." Like, "Verily, Joseph Smith is my prophet." (if you happen to be a Mormon pentecostal). Or, "Verily, Mary hears your prayers" (if you happen to be a Catholic pentecostal). Or, Verily, "I am coming soon to gather you up, and this time I mean soon" (if you're a Rapture-ready Pentecostal, but leave out that last clause).