What do I wear on my first date??

So for context I'm a ‘15 F’ female and I'm going on my first ever (sort of) date with this guy l've liked forever. Let's call him Jake. So l've actually liked this guy for so long, we went to the same school at primary and recently reconnected and have been texting everyday for weeks now. There is some pretty obvious flirting going on between us so l'm 90% sure he likes me. He asked if I wanted to hang out sometime to catch up and I agreed. We live in the same town he lives really close to me, so we're most likely going to walk around, maybe go swimming and get something to eat. I'm really nervous. I think I've got down what im gonna say but WHAT DO I WEAR?? I have this pretty blue mini dress but I don't know if that makes me look too dressed up and maybe l should wear a t shirt and shorts. What do you think? Also, what do I first say when I meet him again? Like I haven't talked to this guy in person for so long so what do I do?? \