Hire a barista...that's about the only thing I can suggest. Unless your customers are extremely forgiving, you may be sabotaging the things you do well by trying to serve espresso drinks with no prior experience. As an example, would you consider opening a bar having never been a bartender? Being petrified comes from fear, fear of the unknown. You need to take charge and be accountable for the success of the business. If you didn't budget for a barista, figure out how you can hire one. If you absolutely cannot afford to hire a barista, then while you are getting up to speed on making the drinks, just be up front with your customers. Fortunaltely, the coffee drinking public, for the most part, is not very critical of the coffee drinks they are sold; proven daily by the success of Starbucks.