What is the verb for 'to will' in French (I will, you will, he will, she will, they will)?

There's no particular verb for it. When you say "I will" + do something, it's a form of future tense. In French, "will" + verb are not two separate words like in English, but simply an addition of several letters to the original French verb. For example, "I will say" in French is "Je parlerai", which is just "parler" + "ai". "I will go" in French is "J'irai". It's actually probably the easiest tense to learn in French.