This page explains how to start a new game of Chrono Trigger.
When you first start a new game, you are prompted to choose your battle mode. You can always change it later in your options menu.
In Wait mode, the battle will pause if you use a menu. This gives you all the time that you need to select an item or choose a Tech, for example, without fear of being attacked.
Active mode is more of a challenge. Enemies can still attack you while you are viewing a menu, but your characters' battle gauges will also increase. That makes you more vulnerable to attack, but it also lets you move your cursor to the desired tech and wait for the character or characters to be ready.
After picking a battle mode (you can change it from the Preferences screen later) name your character (default: Crono).