If it is going to be June, why not go for a cute sundress and pair of high heels? Isn't there any "girly" girl that you know of that loves shopping? How about asking her what you think you should wear? Perhaps she has a fashion tip or two she can spare?
Where is it, what time of day, how was invitation phrased, what's your budget, what do you need (ir do you need shoes etc). If you want suggestions that actually look good your body shape and colors that look good on you would help.