1) How do I write a string of AT Command in Keil?
For example, I want to write this string > ATREMS:000D6F0000D5D5F8,18=00000000 < in Keil.
Is there a way to write this whole string directly? Or other methods?
Not sure really. I can't find out how "/B" could be a summary of this thread. Maybe a dictionary can help you with the meaning of the word "Summary".
Wrint a full string? Yes, you can write a function that loops through the individual characters. Or you can modify the Keil library functions (there are stubs intended to be modified) to let printf(), puts() etc to interface with the serial port.
But you still need to figure out if you want to send your data polled, i.e. where the loop waits character-by-character for the UART to be ready. The UART can only accept a single byte at a time, unless it has a FIFO in which case the size of the FIFO controls the max number of characters it can buffer on it's own.
An alternative, is that you write an interrupt handler and have your program store the data in an output buffer where the interrupt handler can pick up new characters to send when the UART signals that it has room for more data.
Have you looked at the large amount of example code available?