Did you try to do a search on Google?
Cute flats
Nice Summer dress. Cute flats; or shirt wedges. Heels could be over the top. Nice simple jewelry &makeup. Hair well done; curly or straight.
Summery dress + curled hair + cute flats/ flip flops + purse + necklace + simple makeup = perfect!
You can possibly find a nice casual summer dress at a local department store, or even borrow from a friend. If you would rather not wear a dress but would like to still look decent and classy for the occasion you can go for a dressy skirt paired up with nice wedges and a dressy top.
I've been to plenty graduations. A cute summer dress with flats or cute heals would look nice. Just don't go overboard with jewelry, makeup, and hair.