一起【yìqǐ】

一起 means "together". It goes before the verb, not at the end of the sentence the way "together" does in English.

• × 我们吃饭一起 → 我们一起吃饭

跟 + person + 一起

To say who with, put 跟 or 和 + the person before 一起. English "with" comes at the end; here it all comes before the verb.

• × 我去看电影跟朋友 → 我跟朋友一起去看电影

With other adverbs and negation

都, 也 and 常常 come before 一起, and so do 不 and 没.

在一起

在一起 is "together" as a state rather than an action: living together, being together. On its own, 我们在一起 also means "we're a couple".