Mainly
Primarily
adverb
主 means main so when you add に it becomes the adverb mainly.
おもに
Since this word consists of a kanji with hiragana attached, you can bet that it will use the kun'yomi reading. You didn't learn that reading with this kanji, so here's a mnemonic to help you:
You mainly spend your time talking to your master Moe. But you mainly address him like, "Oh Moe The Great!" (おも). That's because he's above you. Imagine yourself saying "OH MOE THE GREAT!" as you walk into his room, bowing low.
主に日本ごをべんきょうしています。
I’m mainly studying Japanese.
ポートランドでは、赤い車と白い車が主にうれています。
Primarily red and white cars are sold in Portland.
彼は研究者で、主にバナナに興味があります。
He is a researcher who is mainly interested in bananas.