I recently opened a business account writing a personal check as "cash" to start the account. Can I write a check to myself.

LLCs need to be adequately capitalized or its members risk having the "corporate veil" pierced, putting their personal assets at risk, and that initial capital contribution does come from the LLC members. An LLC shouldn't pay any distributions to its members until it has repaid any loans and until its financially prudent. Don't try to do your own accounting if you're not a CPA. Your LLC needs a good CPA to make sure it follows the rules respecting its separate legal identity.