It will take you more than a year to really get into your groove.
My advice is don't play ranked until later on in season 8 so you can really get a grasp on the new system they put in and your characters.
A good goal to set for when to start ranked is probably about 200-400 normal games after you hit level 30.
Mid and Jungle are great roles to have control over the map and make plays for your team, from mid you can roam bot if they need help, or even help your jungler in the river or head up top.
Jungler is often the true support of the team, they are the glue that keeps the team together, if you can excel in this role you won't have any problems climbing...
Do you ever rewatch your own games? try it sometimes, take not of your deaths and what you did wrong. Take not of objectives and what you did wrong if you lost them, or what your team did right if you ended up getting a nice steal or something etc etc.
Some people are naturally good at this game due to having good mechanics but most of has have to reflect and improve.
It takes a lot of practice practice practice.