Ch usually says /ch/ like chip. But in school it says /k/, and in chef it says /sh/. If one does not make a word you know, try the next.
The rule
Try /tʃ/ first — it covers the large majority of English words. If that gives no word you know, try /k/, which appears in words from Greek. /ʃ/ is a small French-derived set.
What is really going on
The three values are etymologically conditioned. /tʃ/ is the native Germanic and Old French development and is by far the largest group. /k/ occurs in Greek-derived vocabulary, where ch transliterates chi — school, chorus, chemistry, character, echo, stomach, ache, anchor — and clusters heavily in academic and scientific words. /ʃ/ occurs in later French borrowings that entered English after French had shifted /tʃ/ to /ʃ/ — chef, machine, chalet, parachute, brochure.
Why English does this
All three groups were once the same sound. Latin and Greek ch was /k/; Old French turned it into /tʃ/; modern French turned that into /ʃ/. English borrowed from all three stages and kept the spelling each time, so the spelling records when the word arrived rather than how it sounds.
Position in the word
Beginning of a word — Usually /tʃ/, with the Greek and French sets as the exceptions. For example: chip, chair, cheese, school, chef.
End of a word — Almost always /tʃ/. After a short vowel in a one-syllable word it is spelled tch instead. For example: much, rich, lunch, beach.
Middle of a word — Either, decided by origin. For example: kitchen, teacher, echo, machine.
When this rule works
/tʃ/ is the first attempt in any word a beginning reader meets.
When this rule does not work
The word is scientific, medical or academic — those are where the Greek /k/ words cluster.
The word looks French — chef, chalet, brochure, parachute, machine, chandelier.
Made-up words that follow the rule. A child who reads these has the pattern rather than the vocabulary.
chab, chen, flonch, chust
Exceptions
ch as /k/ — Greek origin: school, chorus, chemist, chemistry, character, echo, stomach, ache, anchor, orchestra, scheme, technology, Christmas. ch transliterates Greek chi, which was /k/.
ch as /ʃ/ — later French origin: chef, machine, chalet, parachute, brochure, chandelier, moustache, Michigan. Borrowed after French had shifted the sound.
Contrast pairs
chin versus shin — The two digraphs children most often swap.
chair versus care — Digraph versus two separate consonant sounds.
chorus versus chore — Same three letters, /k/ and /tʃ/.
Decodable sentences
I had chips and cheese for lunch.
The children sat on a chair in the kitchen.
My chin itches much more than my chest.
Common reading mistakes
school: a child reads /tʃuːl/, where the word is /skuːl/. Default /tʃ/ applied to a Greek-origin word.
chip: a child reads /kɪp/, where the word is /tʃɪp/. The reverse, after school is taught too early.
chair: a child reads /k/ /h/ /eə/, where the word is /tʃeə/. Digraph split into letters.
Do not teach it like this
"Ch says /ch/."
School, Christmas, stomach, ache and echo are all words a young child uses, and all of them break it. Stating the rule without its alternatives means the first time a child meets school they conclude they have read it wrong.
"Ch usually says /ch/. In some words it says /k/ and in a few it says /sh/. Try /ch/ first, and if that is not a word you know, try /k/."
Teacher note
Teach /tʃ/ alone for several weeks — it is the overwhelming majority and a child needs it automatic. Introduce /k/ as a named group when school, Christmas and stomach start appearing, and give children the flexing strategy explicitly: try the first sound, listen for a word you know, try the next. That strategy is more valuable than the ch facts themselves, because it is what every variable grapheme needs.
Teaching sequence
ch as /tʃ/ at the start: chip, chin, chop, chest.
ch as /tʃ/ at the end: much, rich, such — then contrast with tch in catch, match.
Introduce flexing: try sound one, does it make a word?
The Greek /k/ group, named as a group: school, Christmas, stomach, ache, echo.
The French /ʃ/ group, last and briefly: chef, machine.