The German digraph »ch« is never pronounced like »h«. English has no sound like German »ch«. Only some Scottish words like "loch" (as in "Loch Ness") contain this sound, but only in Scottish pronunciation, not in standard English pronunciation.

• after velar vowels (aka "dark vowels" or "back vowels") like »a«, »o«, »u« or »au« (Examples: *»Dach«, »Loch«, »Buch«, »Rauch«) if »ch« is not part of the diminutive suffix »-chen«.

• After any vowel that was not listed before (»e«, »i«, »ä«, »ö«, »ü«, »y«, »ei«, »eu«, ...). For example: *»Pech«, »ich«, »Gespräch«, »Küche«, »Psyche«, »Teich«, »euch«, ...)
• After any consonant, for example *»Milch«, »durch«, »manch«
• At the beginning of a syllable inside a word, including the diminutive suffix »-chen« for example »Kirche«, »welcher«, »Frauchen«, »Autochen«

If there is an »s« before »ch«, and both belong to the same syllable, then it's no longer a digraph but the trigraph »sch« that is pronounced like the English digraph »sh«:

German: Englisch
English: English

If the »s« before »ch« belongs to another syllable, then its [ç], as described before (*»Häuschen«).

If an *»s« comes after »ch«, then it will be pronounced like »k«, and together with the »s« this gives [ks] which also is the standard pronunciation for the letter »x«. And so, the two German words »sechs« (Engl: six) and the word »Sex« (engl: sex) have an identical pronunciation in German.

In regions where Bavarian dialects are spoken, »ch« at the beginning of a word are pronounced like »k«:

• Chemie = [keˈmiː]
• China = [ˈkiːna]

In other regions (I believe in the north, where Low German is spoken), it becomes the same sound as »sch«:

• Chemie = [ʃeˈmiː]
• China = [ˈʃiːna]

• Chemie = [çeˈmiː]
• China = [ˈçiːna]

But geographic names of places in German, Austria, and Switzerland are pronounced equally everywhere, and in which way it should be pronounced depende on where this place is located.

So, Chiemsee is in Bavaria, where Bavarian dialects are spoken, so the ch is pronounces like k: [ˈkiːmzeː]. Even if people from Hamburg or Essen pronounce this name. For Chemnitz its the same.

Sorry, I can't help you with Cham becasue I don't know it.

¹ _{The Wikipedia articles have links to soundfiles, so you can hear the sounds}

The German digraph »ch« is NEVER pronounced like »h«. English has no sound like German »ch«. Only some Scottish words like "loch" (as in "Loch Ness") contain this sound, but only in Scottish pronunciation, not in standard English pronunciation.

»Ch« can be pronounced as [x] or [χ]

• after velar vowels (aka "dark vowels" or "back vowels") like »a«, »o«, »u« or »au«, if »ch« is not part of the diminutive suffix »-chen«.
  Examples:

  Dach, Loch, Buch, Rauch

»Ch« can be pronounced as [ç]

• After any vowel that was not listed before (»e«, »i«, »ä«, »ö«, »ü«, »y«, »ei«, »eu«, ...).

  Pech, ich, Gespräch, Küche, Psyche, Teich, euch, ...

• After any consonant,

  Milch, durch, manch, ...

• At the beginning of a syllable inside a word, including the diminutive suffix »-chen«

  Kirche, welcher, Frauchen, Autochen

»Ch« combined with preceding »s« as part of »sch« is [ʃ]

If there is an »s« before »ch«, and both belong to the same syllable, then it's no longer a digraph but the trigraph »sch« that is pronounced as a voiceless postalveolar fricative (IPA-Symbol [ʃ]) like the English digraph »sh«:

German: Fisch = [fɪʃ], Schuh = [ʃuː], Busch = [bʊʃ], ...
English: fish = [fɪʃ], shoe = [ʃuː], bush = [bʊʃ], ...

If the »s« before »ch« belongs to another syllable, then its [ç], as described before (»Häuschen«).

»Ch« combined with following »s« as part of »chs« is [ks]

If an »s« comes after »ch« within the same syllable, and if this »s« already is part of the stem, then it will be pronounced like »k«, and together with the »s« this gives [ks] which also is the standard pronunciation for the letter »x«. And so, the two German words »sechs« (Engl: six) and the word »Sex« (engl: sex) have an almost identical pronunciation in German. Only the »s« at the beginning of the word is pronounced differently in northern parts of German sprachraum. In southern regions both words sound exactly identical.

Dachs = [daks], Fuchs = [fʊks], Lachs = [laks], Wuchs = [vuːks], Gewächs = [ɡəˈvɛks]

But if the »s« after »ch« is attached because of a grammatical modification (i.e. if it is not part of the stem), then »ch« is pronounced as a fricative ([x], [χ] or [ç]) as described above

mittwochs = [ˈmɪtvɔxs], (des) Bereichs = [bəˈʁaɪ̯çs]

So, part of the stem: »der Buchs« (box tree, boxwood) = [bʊks]
Not part of the stem: »des Buchs« (genitive form of »Buch« = Engl. book) = [buːxs]

»Ch« at the beginning of a word can be [k], [ʃ], [ç] or [t͡ʃ]

In regions where Bavarian and Alemannic dialects are spoken², »ch« at the beginning of a word is often pronounced like »k«:

Chemie = [keˈmiː], China = [ˈkiːna], Chirurg = [kiˈʁʊʁk] or [kiˈʁʊʁg]

In other regions (I believe in the north, where Low German is spoken²), it becomes the same sound as »sch«:

Chemie = [ʃeˈmiː], China = [ˈʃiːna], Chirurg = [ʃiˈʁʊʁk]

Chemie = [çeˈmiː], China = [ˈçiːna], Chirurg = [çiˈʁʊʁk]

But geographic names of places in German, Austria, and Switzerland are pronounced equally everywhere, and in which way it should be pronounced depends on where this place is located.

So, Chiemsee is in Bavaria, where Bavarian dialects are spoken, so the ch is pronounces like k: [ˈkiːmzeː]. Even if people from Hamburg or Essen pronounce this name. For Chemnitz it's the same. (When Chemnitz got it's name, the local dialect used [k] for »ch« at the beginning of a word. I'm not sure, if they would pronounce it today with [k] if it was a new word, because I don't know how people in Chemnitz now pronounce words like »Chemie« or »China«)

Sorry, I can't help you with »Cham« becasue I don't know it.

But for many other words, there are no regional differences. They are pronounces equally everywhere:

Chef = [ʃɛf] or [ʃeːf]
Champignon = [ˈʃampɪnjɔŋ]
Charme = [ʃaʁm]

Chor = [koːɐ̯]
Chaos = [ˈkaːɔs]
Christ = [kʁɪst]
Chrom = [kʁoːm]

Chip = [t͡ʃɪp]
Chart = [t͡ʃaʁt] or [t͡ʃaːɐ̯t]
Chat = [t͡ʃɛt] or [t͡ʃæt]

Best strategy to learn the correct pronunciation

The best strategy to learn the correct pronunciation of a word is to learn it for each word separately. Your brain will find the correct patterns after a while on it's own, just by learning many different words.

Learning German pronunciations is much easier than English pronunciation, because English has much more and much more complicated rules for pronunciation than almost any other language, including German.

¹ _{The Wikipedia articles have links to soundfiles, so you can hear the sounds}
² _{For the geographical distribution of German dialects please read this article.}