You should try as many champions as you can and find the role that you like. Even though most people at higher elo tends to know a bit of every role. they won't main more than 3 out of 5 roles. Some roles such as jungler or support is completely different from the others, thus really important to learn since you can't pick your favorite role in normal or ranked every single time.
Last hitting is a good thing to practice on, other things such as zone control, poke, gank and the attempt to take the map control are also really good to start practicing on.
As for runes set, if you want to jungle pre-20, you need t1 runes and a leash most of the time. Junglers should not jungle without a leash since it will be a lot slower and the true effectiveness of having a jungler will not be as clear as no jungler. Once you are level 20 you should move on to t3 runes
If you seriously don't want to buy t1 runes, then you can try to jungle with safe junglers such as warwick and udyr. Both of them have nice built in sustain.
*Note: Don't buy t2 runes
Generally for builds there are 3 main idea
1. Cost effectiveness
What this means is that no matter how glass cannon you want to build your ranged AD as, you should still leave an item for defensive purpose, since a dead AD cannot do any damage to the enemy
At the same time, even though a bruiser can go really tanky by just building warmogs, GA, banshee and more, it is advised not to do so, since the enemy will be able to ignore a full tank that is not a threat to them.
Finally, builds should be cost effective. One good example will be the balance between health and resistance. In order to achieve the most effective health (eHP), you must find a balance between building health and resistance items. And even then, if the enemy have true damage or % health scale damage, all the math will be needed to recalculate again. This takes a lot of experience but you should always pay attention to.