Learn Turkish: Yarın şehir turuna çıkacağız. - We will go on a city tour tomorrow.

The verb root is çık (to go out). We add the future tense suffix -acak to get çıkacak. When we add the "we" ending -ız, the k is followed by a vowel. In Turkish, a k between two vowels changes to a soft g (ğ) to make pronunciation smoother. So, çıkacak + ız becomes çıkacağız.

Why did we use the -acak form of the future tense instead of -ecek here?

The vowel harmony rule for the future tense depends on the last vowel of the verb root. The root çık contains the back vowel ı. Because it is a back vowel, we use the A-type suffix -acak. If the root had a front vowel like in gel (to come), we would use -ecek to make geleceğiz.

Why does the sentence use çıkacağız (from çıkmak) for "go" instead of a form of gitmek?

In Turkish, the verb çıkmak literally means "to go out" or "to go up", but it is the natural verb to use for embarking on journeys or activities. You use çıkmak for set phrases like tura çıkmak (to go on a tour), yola çıkmak (to set off), or tatile çıkmak (to go on vacation). Using gitmek here would sound unnatural, as if you were just commuting to where the tour is.