Old t-shirt and old pants/shorts that you don't care about getting disgisting and ripped. Boots, hat, sunglasses, and maybe sunscreen...u might need that. And waterrrrr, always be careful working in the sun for long periods of time, heat exhaustion, etc is dangerous. Good luck! Maybe you'll fall in love with the farmer's daughter! :) link
Really depends on what kind of farm. Obviously clothes that you don't care about. NO SNEAKERS. wear boots (not rainboots though), that can get covered with mud but have enough support for running/ fast walking. There is a reason that farmers wore overalls, but if that's not "you" or you don't have any, that's totally fine. If its going to be hot, wear something light (as it will be a LOT of physical work). wear a plain shirt and a hat doesn't hurt. Loose and comfy old jeans are good, and a plain shirt will be fine (I think). Again, know what kind of work you are doing and how dirty you are going to get.
I used to work at a horseback riding stable and did a lot of manual labor outside. I always wore jeans, boots, a long sleeve shirt, and baseball cap. You don't want to be out in the sun all day exposed, so even if it's just a thin button up shirt over a tee/tank or something it's good to stay covered. Have sunblock on as well, and work gloves if you got them.