What kind of clothes should I wear?

your outfit seems fine but NO running shoes outside of the gym yuck I hate those. maybe try wearing some timb's or something

My suggestion is a nice v-neck. I think they always look good on guys.

It sounds like you dress cool :) Do you wear old school vintage style running shoes, because those are pretty cool.
I really like the beach thing, too, though.
I think it's basically a bit looser shirt with cargo shorts/pants. Maybe try skate shoes or Vans slip ons with it if you're not going to wear sandals. But the vintage style sneakers would still work. Beachy jeans are kinda loose and worn or a little ripped/frayed...
And there's also the long-sleeved thermal underneath the t-shirt.
I personally love long or shaggy hair on guys, but not everyone does.

There's some pretty cool Plad Shirts that work well with the whole surfer-beach-boy-stoner look I think you're going for. Works well for us skinny guys too (I'm 6'1, 150 lbs) cause we can wear a t-shirt under it, and have the top two buttons popped out to show your chest a bit, and have the last button on the bottom open so it makes a sorta V at the bottom. And since they're a bit baggy it kind of emphasizes your height without making you look super skinny. Oh and if you wanna show off your arms a bit you can role up the sleeves to a little below your elbow (two ways to do that, either fold them twice and scrunch them up the rest of the way, or just keep folding them but make sure the cuff doesn't get too thick cause that just looks wack).

Err so I totally didn't set out to write a paragraph...
If it's unclear just say so and I'll take a picture as an example

First off, get rid of the running shoes, those are for exercise. Second, what kinda beach boy, like the stoner laid back guy, the fashionista surfer a.k.a. hollister, etc.
Like if you like the laid back weed smoking look, grow out your hair get some loose fitting tank tops read surfing magazines and just copy those guys.
Fashionable surfer looking guy- get zach efron haircut and just buy whatever you see at hollister.