In your sentence, qui is the subject of the subordinate sentence, and qui is a pronoun that replaces "moi", therefore "vouloir" must be used at the first person ==>
C'est moi qui veux !
Note that qui will also je either masculine/feminine depending on what it replaces, e.g.
C'est elle qui est venue.
And qui replaces the nominal group that is directly before it, therefore you say (as mentioned by @Distic)
C'est moi la personne qui a faim.
qui = la personne in this example
As a remark, there is no such consensus in English: it seems that you can write both
It's I who want
It's I who wants
even if the first is more frequent.
But in French, it would definitely be a mistake to write
C'est moi qui veut (**WRONG**)