Originally, From The Beginning, By Nature
adverb, の adjective
It's the origin of all origins, so it's the way it was originally, from the beginning, or by nature. You get the idea. Note that this is frequently written in just hiragana, but you'll encounter the kanji version too!
もともと
This is just the kun'yomi reading you learned with 元, but twice, so you should be good to go!
このホテルは元々アパートだったんです。
This hotel was originally an apartment.
あのモデルさんって、元々はデザイナーだったんでしょ？
That model used to be a designer, right?
元々この家にはトイレもキッチンもあったんです。
This house originally had a bathroom and kitchen in it.