If you want it to read as “a hundred,” you simply write out in letters “a hundred” (without the quotes).

If you want to say “one hundred,” you can write 100 or “one hundred” (without the quotes). Typically shorter numbers in the single or double digits should be written out with letters and then switch to numerical representation above that to be considered “correct” by some academics.

Finally, if you want to write “a one hundred,” simply write “a 100.”