A good start is simply knowing the frequency of letters.
Most Frequent Letters:
E: Around 12%
T: Around 9%
A: Around 8%
O, I, N, S, R: These letters also appear with high frequency.
Least Frequent Letters:
Q, J, X, Z: These letters are used much less frequently than others.
And common endings like
“Ing”
“tion”
“ed”
Etc
Today’s is a good one to start with!