How can I tell whether "c" should be pronounced "s" or "k"? I always get confused and pronounce it like "s" because it looks like russian "с".
C can also be pronounced as sh sometimes, but this is rare. It is most often found in adjectives ending in -cious, such as conscious, precious, ferocious, etc, and also in adjectives ending in -cial, such as special, artificial, crucial, social, etc. One major exception where c is pronounced as sh is the word ocean.
There are many other exceptions to the above rules. You will learn them as you go.
As Jim points out, an excellent rule of thumb is "Except for ci, ce, and cy, it's pronounced as a k." With the exception of ch, of course.
There's also one or two words in English that begin with cn, in which case the c is completely silent. But they're very, very rare scientific words. There's also at least one that begins with cth, in which case the c is again silent.
Pretty much if c is followed by i, e, or y it's pronounced like s. Otherwise it's pronounced like k (except sometimes when it's followed by h and is pronounced like chalk or children)
Do you want to know if "c" and "k" sound the same? Think of some words like cliff, kitten, kettle, computer, camera, like. Pronounce these words and you will notice that both "c" and "k" are pronounced the same as "k".
It is the same case if "c" and "k" are put together and are pronounced as "k" - click, pluck, tickle, lucky, kick, yuck, wicker (and many words that have "c" and "k" together).
The difference between "c" and "k" is:
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As for the letter "k", it has no other pronunciation, just "k". But be careful when the letter "k" goes with the letter "n" - know, knife, knowledge, knight. In this case, "k" is not pronounced.