Because Wikipedia content is licensed under the GFDL and CC BY-SA, all edits must be kept for attribution purposes, and so your account cannot be deleted. You do, however, have the right to vanish, which you can exercise by (1) requesting your user page (found at Special:Mypage) and subpages be deleted, by adding the {{db-userreq}} template to them; (2) requesting that your account be vanished at Special:GlobalVanishRequest. If granted, your account will be irreversibly vanished. The "right to vanish" does not mean anyone has the right to a fresh start under a new identity. Anyone who wants to continue editing should request a change of username instead so edits can be reattributed.