Use the key levels as a way to grow. Which is what youll want to do to have your IO update each day anyway to be accepted into higher and higher keys.
You can just start running +2-5 really easily. Get +5 in all dungeons, get +6 in all dungeons, +7 in all dungeons and so on til you’re running 17’s like a champ.
Download MDT addon, you can use this to inspect the abilities of most trash mobs in the dungeons, pretty helpful to get a rough idea of what exactly is happening and what abilities are really bad. Between this and actually running just a lot of dungeons you start to learn which packs are dangerous and unavoidable, or which packs are really dangerous if people dont stun/interrupt, etc etc and you start to feel out when to pre-hot, what to hold cooldowns for, when to stun for your teammates if they’re bad pugs etc.
There’s also some good videos on youtube you can watch of higher keys with commentary. Yumytv is a really enjoyable guy to watch, though he plays shaman more now. He has some vids where he watches someone else’s run and gives feedback, if you’re ever bored and interested watch some of these. 
Biggest tip I can give is to do your best not to feel discourage or like an imposter when even a +4 ends up becoming impossible due to people completely failing mechanics, or not interrupting, or not handling affixes properly, none of this is your fault and it can be REALLY hard without experience to tell who’s fault it really is.
I use the ElitistHelper addon and (/eh output self) command to have it whisper me privately who is hit by AVOIDABLE damage and how much, this helps a ton to see what’s actually happening.
#Knote1805 you can add me if you ever have questions.