我 正在 买 饮料 => 我 要 买 饮料

put it in a simple way, you just add 要,将, 准备 or 将要 in front of verb. examples: original sentence: 我看一场电影. future tense: 1. 我要看一场电影。 2. 我将看一场电影。 3. 我将要看一场电影。 4. 我准备看一场电影。 Using which one depends on the context. Sometimes, they all work in a sentence like the example, but sometimes, you'd better pick a specific one. It takes time to cultivate the language feeling.

I want to help you ,ban , i cant give you a reason in English