Trip.com is a reliable way to buy tickets, you can request an aisle, window or middle and get allocated a seat depending on availability. You may have to queue for quite a long time to collect them, and anyone jumping the queue is likely to be military or ex-military and that is allowed. Expect to queue multiple times, you go to the window with your passport and the Trip.com printout and they will recognise it and issue your tickets. Major stations have ticket offices clearly marked in English with an obvious symbol. If you have to buy at the desk, come prepared with the train you want written down. They will use a calculator to show you the price.
Even first class on the high speed trains can be quite inexpensive by our standards. The 3+2 standard class I took from Beijing West to Wuhan was spacious enough, even in the middle seat. There is enough English on displays, platform indicators etc to get around, and your trains will have an ID in the Latin alphabet and Arabic numerals at the very least. They have a mixture of Siemens, Bombardier, Alstom and Shinkansen-derived as well as locally designed high speed stock. I haven't travelled on the classic stock and unless you're really scrimping or want the authentic experience, I wouldn't!
As with anywhere in China, the carriages are quite noisy with people watching videos on phones and piped music and adverts. It's quite irritating although you get used to it. I could tolerate it better than I would a noisy phone user in the UK as you have to accept the culture and you take it and observe it differently. Noise cancelling headphones would be a good idea for longer journeys.
The food on train is pretty ropey looking, take your own, pretty much anything can be bought from major stations. Many people bring noodles and make them on-board with boiling water from the free dispensers in each carriage. You may drink alcohol on trains.
Shanghai and Beijing metros are extremely clean, modern and relatively easy to navigate, even having English announcements and displays. You won't need to learn any Mandarin, and I wouldn't even try, there's little chance of getting the dialect anywhere close anyway. Know what please and thank you are, and use the Google Translate app for menus etc.
Carry your passport everywhere and expect to have to show it to officials on a regular basis, but I would say Beijing has become much much friendlier than it was when I first visited about 8 years ago.