If you own a property as tenants in common, this means that it belongs to you and the other owner(s) jointly, but that you all also own a specific share of its value. It is up to you to decide how much each share will be.
You can give away, sell or mortgage your share. If you die, your share of the property does not pass automatically to the other owner(s). You can leave your share to whoever you like in your will.
More information about tenants in common can be found in Land Registry leaflet Joint Property Ownership http://www1.landregistry.gov.uk/assets/library/documents/public_guide_018.pdf ).