How you store/access the strings depends on the programming language you use. Are you going to use Basic, C, or ASM?
Start with very simple code. ie: Write a routine that sends one character/byte at a time out the USART. Then simply send the ASCII for 1 (0x31), delay at bit, and then send an ASCII line feed (0x0A) and, if all is working right, you'll hear it say, "one".
After that bit of inspiration, look at storing complete phrases in ROM and rewriting the routine to send the entire string in sequence.
Only after that is all debugged and working, should you proceed with the rest of the coding for stimulus decoding etc.
ie: Make it modular and do it one small step at a time.