Camiseta = T-shirt Camisa = shirt (like a dress shirt or just a long sleeved one)
Camiseta is a t-shirt, undershirt, or sports jersey. Camisa is a shirt such as a polo or button-down. Camisa also has some non-clothing meanings.
FULL DISCLOSURE: Native English speaker - US, Southern Appalachian dialect. Other uses of English may vary. Advice about Spanish should be taken with a grain of salt.
T-shirt is the most common spelling (and the more logical, because of the T shape), but they do accept t-shirt now in dictionaries, and also tee-shirt, tee shirt (Cambridge only accepts T-shirt and tee shirt)
tshirt and t shirt are wrong spellings.
You can already use T-shirt (recommended spelling, the most common, and the most logical, because of the "T" shape), tee shirt, tee-shirt, and t-shirt. https://grammarist.com/spelling/t-shirt-tee-shirt/
Yes, the "T-" part is strictly an English matter. Often Spanish uses "-ita" as the diminutive form, I believe.