There are a few options. Usually you will use the emphatic pronoun moi (also called the disjonctif).

For example, you could say any of these depending on how verbose you want to be:

Moi.

C’est moi.

C’est moi qui décide.

The emphatic pronouns are je / moi, tu / toi, il / lui, ils / eux (elle, nous, vous, elles don’t change).

You could also just use je but it would be less likely to appear without the emphatic moi :

Moi, je décide.

Je décide.

If you want to say something like "I do", you have to use le (literally "I do it"). This is true every time you want to replace a verb with "do". The same thing about moi applies here too.

(Moi) je le fais.

You can’t reply with just oui because this is an open-ended question, not a yes/no question. The following exchange makes no sense in either English or French.

Qui décide ce qui est beau ? Who decides what is beautiful?

Oui. Yes.