Find: Summon Ramuh Materia

Enter the Gold Saucer and talk to the person you want to join you, then follow Barret into Wonder Square where you meet Cait Sith. He foretells events and joins your group. After this, go to Speed Square and talk to the guy in the orange shorts. His name's Dio, and he owns the Gold Saucer. Next, go to Ghost Square and read the Turtle’s paradise Flyer No. 3 on the wall (it says SHOP). Next, head to the Battle Square and find a dead guard.

Once in jail, examine the dead guy, then run to the next screen. Go all the way to the bottom and right to buy items, then go in the little shack on the right side to find Barret. Form a party, then leave the shack. Run up back to the first screen, and leave the prison on the upper right exit. Keep running right after you leave, and you will end up in Dyne's junkyard. Follow the brown gravel road up and to the right. After some dialogue, you must fight Dyne.

After you win, Dio will give you the buggy, which lets you cross deserts and shallows in the rivers. Take the time to learn the Enemy Skill Aqualung from the Harpys before leaving. Just drive around in the sandpit surrounding the Gold Saucer until you hit one.