One big issue is that I really have no idea what to say to other people, and that holds true even if I'm writing. I had to write a cover letter and a thank you letter in class and I gave up after three sentences even though it was supposed to be three paragraphs. I literally have nothing to say. Or more accurately I have nothing to add beyond the absolute minimum necessary to make a point.. What should I do?
Okay, cover letters can be difficult to write, especially if you don't know who or what you are applying to.. Maybe it helps to imagine a great job (or find a really good job ad online) and research the company (what they want etc) and pay close attention to the job ad that says what they want.. then it's easier to write.. There are lots of tips on how to write great CVs and cover letters online.. Read and research.. Yes, a lot of that stuff really works.. (Use some common sense too..)
Also for a thank you letter, 3 paragraphs seem quite a lot - you can still think of something you were or would be really thankful for and then describe it and thank for it genuinely?
About what to say to other people: it helps to get to know them and then talk about their interests or their life etc - they absolutely LOVE this and think you're a great person to talk to!!
You just need to steer conversation and pose some basic questions to start with and nod and 'uhm' and 'aha' sympathetically or recap or provide some feedback or some more questions and often they will absolutely LOVE talking to you!!
(The idea is to be mostly non-judgemental and open-minded, learn from them, build on what you both are interested in, stay away from strong judgements or disagreements or controversial issues unless absolutely necessary.. Of course stand strong on the really important things, it still helps to be diplomatic about it if you can.. Yes, you can learn being diplomatic too..)
It helps if you are genuinely interested in people.. Otherwise, try to find more interesting people

or such that you have more in common with..