The main thing I'd advise is to talk to a lawyer (and, possibly also a tax accountant).
Any time you bring somebody else into your business, you're going to want to make sure you have a written
agreement ("handshake deals" are definitely
not sufficient) that spells out each party's responsibilities and the scope of their authority. It should also address how to handle things should one partner decide to exit the partnership, and what happens if one of you shuffles off this mortal coil prematurely. For instance: can you leave your share to whoever you want in your will, or does your business partner get a say in who his new partner(s) will be?
Any decent lawyer will ask a bunch of questions to make sure you and your partner have considered all the contingencies and that they're all addressed in your partnership agreement.
You may also want to have a chat with a tax accountant. Going from a sole proprietorship to a partnership will potentially have some tax implications for you, so you'll want to be sure you're prepared and that your business has been organized in such a way as to minimize the impact.
Good luck! Hope this helps.