Learn Turkish: Gelecek yıl emekli olacağım.

This is the future tense for "I will become". The base is olmak, plus the future suffix -acak, making olacak. When we add the "I" suffix -ım, the hard k is sandwiched between two vowels and softens to ğ. So, "olacakım" becomes olacağım. This consonant mutation always happens for the "I" and "we" forms in the future tense.

How does the verb for "retire" work here?

The word emekli actually means "retired" or "pensioner". Turkish often pairs adjectives or nouns with the helper verb olmak (to become / to be) to make an action. So emekli olmak literally means "to become retired".

Doesn't gelecek mean "it will come"? Why does it mean "next" here?

You are exactly right! Gelecek is the future tense of gelmek (to come). In Turkish, future tense verbs can sometimes be used as adjectives. Here, gelecek describes yıl (year). So gelecek yıl literally translates to "the year that will come", which is how you say "next year".

Will I hear people pronounce every letter in olacağım?

Usually, no! While you must write it as olacağım, Turkish speakers often squish future tense words together in everyday conversation. You will very commonly hear this pronounced as "olucam". It is good to know the correct spelling for reading and writing, but don't be surprised when you hear the shorter version.