The rule
One rule that fixes a lot at once: sch is a single sound, the English "sh". And at the start of a word, st- and sp- are pronounced that way too — Stadt is "Shtadt", sprechen is "shprechen".
sch · st- · sp-
| Schmerzen · Schulter | sh |
|---|---|
| Stadt · Straße · Student | SHt- |
| sprechen · Sport · spät | SHp- |
| but mid-word: Post, ist | plain st |
Only at the START of a word. „Post" and „ist" keep a plain st — which is why „Stadt" and „Post" sound different despite the same letters.