Dedication
A dedication is a promise you make to your own practice: that you will study, or observe the Wheel, or keep going for a set period. It is personal, it needs no witnesses, and it confers no rank.
Self-dedication for solitaries
A common solitary custom, popularised by writers such as Scott Cunningham from the late 1980s, is a self-dedication ritual, often after a period of study. Typical shape: prepare, state what you are committing to and for how long, mark it somehow, write it down. Optional, entirely.
Initiation
Initiation in British Traditional Wicca is conferred by an existing initiate within a lineage, in a coven, and carries oathbound material and degrees. It is a formal admission into a specific group, not a general spiritual upgrade.
You cannot self-initiate into a lineage
You may absolutely self-dedicate. You cannot make yourself a Gardnerian or Alexandrian initiate, because the whole meaning of those terms is the lineage of who initiated whom. Anyone selling you a lineage initiation online is selling you a certificate.
Nothing is required
A great many Wiccans have never done either. Practice without a ceremony marking its start is not provisional.