Hello, and welcome to Android Central!
I definitely reiterate what B. Diddy said above; especially with regards to *ALWAYS* disabling your mobile data while it is not in use, and with making sure apps will not be allowed to run on your device, even when your data is turned on. For example: Netflix. My Nexus 5 (when I had it) would automatically switch from Wi-Fi to mobile data (another no-no for some people, especially me); and so, if I was, say, watching Netflix, my device would be running data instead of consuming my Wi-Fi data, which is NOT what I wanted. I went over my data limit that month and so now I am meticulous about my data usage. Netflix and YouTube can use a lot of data; so be sure to prohibit them from running while you are connected to a mobile data source if you so wish.
Also, disabling your Wi-Fi/mobile data while you are sleeping are good for conserving battery (it works for me; though some say the effect is negligible) and the amount of data that can be used just while the phone is idle (i.e., social media feeds refreshing every hour or so, etc.).