Hi! My name is Cass and I'm new to gardening. I thought it would be easy, but it's a lot harder than I anticipated.

I noticed my beefsteak tomato plant had some yellowing leaves that eventually turned brown. At one point I had old and dying branches in the middle of the plant falling off, but the rest of it continued to grow. Today I have these yellowing, and then browning, leaves on my beefsteak. I've also included a picture of my cherry tomato plant's leaves which seem to have a similar issue, but to a smaller extent. I don't know what's up with my cucumber leaves.

I'm worried I have a fungus in my plants, in which case I think I have to give it up and try again next year. Strangely these two tomato plants sit beside two green pepper plants and my pepper plants appear unaffected. I admit that despite the heat (90+ degree temps in my neck of the US) I've only been watering everything at night. I realized my plants weren't getting enough water because one tomato had blossom-end rot. Today I started watering in the morning and at night. My plants get half a day of sun, half a day of shade. I bought a ph testing kit that will arrive Saturday.

If you have recommendations, please let me know! This is a lot harder than I thought. I feel like I should have tried a succulent first.

Those tomato leaves could signal a few things, some easy to fix and other much less so. Before proceeding any further one thing I need to know: have you noticed any white round lesion/spot on the tomatos themselves?
As an aside, lack of water does not cause Blossom End Rot (BER). Watering unbalances and lack of calcium in the soil do, and some cultivars (San Marzano and many of its hybrids to say one) are extremely vulnerable to it. BER is cured by a combination of soil amendment (since you grow in containers you can use liquid calcium boosters), regular watering schedules and growing BER-resistant cultivars: no point growing San Marzano in an area prone to almost daily thunderstorms.

Now let's get to that cucumber: it's almost textbook overwatering. The cure is simple: cut the water.

Lower leaves are dying off from lack of sunlight and getting water containing bacteria and fungal spores getting on them. It happens to everyone everywhere who ever grows tomatoes. If your weather doesn't freeze, your tomato will eventually die from blight or something similar. Just clip off the dead leaves and spray a copper fungicide once a week.

Hi and thank you so much, both of you! Since it's going to storm here for a few days I'm not going to water any plants. I will get some copper fungicide this weekend too. I'm glad that despite what's happening to my Roma tomato plant, my cherry tomato plant is in a much better state. I hope my Roma tomatoes start turning red soon; I get a few cherry tomatoes a day, and a good cucumber every week or so. My peppers grow like weeds. Huge! And delicious!

Cherry tomatoes generally seem pretty hardy, seemingly closer to their natural original genetic stock.
My one Roma last year died first. Have tried a couple times with Roma- I am not a fan of Roma as I got so few goodsized ones like grocery store size.