It really depends on what the rest of the sentence says. You can start the sentence with either "to ensure" or "ensuring." The gerund (v+ing) is more common as the subject of the sentence. "Ensuring complete customer satisfaction takes teamwork." The V+ing can also be used in a participial phrase, but I think it would sound funny with the verb ensure. "Ensuring good customer satisfaction, the clerk stayed on the telephone with Ms. Smith for more than two hours." The infinitive (to +verb) can also be a subject, but it really isn't as common. The infinitive can also be a phrase that shows purpose: "To ensure complete customer satisfaction, we will provide full refunds on every item." This means: "We will provide full refunds on every item SO THAT customers are satisfied."

Yes, it does depend on what follows "to ensure/ensuring". If it's a gerund, you're talking about the action of "ensure": "Ensuring our success is the most important thing." So the sentence is about the action of making sure. It is also possible to write, "to ensure our success is the most important thing", but that's not a very common form. Still, you are making an action the subject of the sentence. But if your sentence is something like "to ensure our success we will take these actions", that's two clauses you could swap over: "we will take these actions to ensure our success". So now you see that "to ensure" works as a verb, and is not the subject.

it depends on what you want to say to tell you the truth, it's me who took your money. watching football matches is exciting.