wear a long enough top so you don't show your butt crack when you're bending down to pray/worship/prostrate.
i'm serious! i find bare bottoms distracting.
i think 'long' shorts that cover the knees are ok, but don't wear a really tight top that shows your muscles/fat tummy (delete as appropriate!)
short sleeves are also ok, but vest tops are too small, don't distract/tempt your sisters!
if the people in your church are low social class, they might wear casual long trousers (or long shorts) and t shirts.
if they are high social class, they will be in suits.
in my church, they are a big mix, but most people reserve suits for weddings/Christmas/easter(Pascha) etc. some people come in t-shirts and jeans and others are smarter.
if u have no clue, i recommend a clean but not ultra-smart long sleeve shirt (you can roll up if folk have short sleeves) and long trousers that are not ultra-smart. that way you won't embarrass any poor people, but also you will look smart enough. you could always keep a tie in your bag in case of emergency! from what i have seen in the news, most of usa is very hot right now, so i don't think a jacket is a good idea unless there is fierce air conditioning (we don't need air conditioning in uk!) - maybe a t shirt in your bag is a good idea, so you can pop to the bathroom and put it under the shirt if there is air conditioning.
for ladies, i would recommend the same, but a skirt, not trousers/pants. in my church several ladies wear trousers, but, from what i have read, most OCA churches are not keen on trousers. look at your self in the mirror before you go and try to bend down, and if you can see clearly the outline of your beautiful butt, get a looser skirt. skirts should reach to the knees so u don't distract the guys, and don't show your breasts/cleavage, it can be really distracting. a few women in my church wear dark leggings and long tops that reach nearly to the knees, no-one seems to mind this. it is generally not a good look on guys.
visiting ladies are not expected to wear a headscarf - if u are not sure, keep one in your bag and decide when you get there if you feel comfortable with it.
i hope this helps! the main thing is to go and learn from God. He sees yr heart when He looks at u, not yr clothes!