In English, we have a variety of ways to say "approximately". There are several in Mandarin as well.
大概 (Dàgài) is pretty common
大约 (Dàyuē) is also pretty common
差不多(Chàbùduō) is literally like "less not more" and is often used for "about" but it's closer to "almost"
But the one I wanted to highlight today is a less obvious one:
左右 (Zuǒyòu) is literally "left right" but is used for "approximately" or "roughly".
It's a great example of how Chinese often uses opposites in a simple way.
I'll write more soon on the best methods for learning Mandarin. If you want to get a taste for it in the meantime though, my current favorite site is iTalki, and my favorite teacher by far is Amy He. If you decide to try it, click here and it's cheaper for both you and me.