Like Arcinex said, play normals for practice.
Have at least 2 (preferably at least 3) champions of each role (top, jungle, mid, support, ADC) that you can play consistently decent with.
Try to pick champions that you might be able to carry a team with, rather than more passive champions. For example, I like
for jungle because he's very tanky, can pick people off, has good damage, etc.
It's a good idea to pick champions and counter-counters to them... e.g. if you know you struggle against
as
, consider learning a counter to
such as
.
It always helps to have a friend to duo queue with as well -- someone you can rely on and work well with. If you can voice-chat with them, even better!
Also, always remember to be kind to your teammates. I've had games where I've just been struggling and getting really angry, but one kind soul on our team just stays positive and encourages us to do things right in a nice way. Be that guy. Don't be the guy that rages because he doesn't get to play the role he wants, or the guy that blames his teammates for everything, or the guy that ignores his teammates because they made mistakes.
Finally, WARD! Always ward! Tell your teammates to ward! Wards win games! Remind your teammates (and yourself) to upgrade the stealth ward trinket -- the upgraded version gives 1 free ward every 60 seconds and the upgrade only costs 250g (requires level 9). Wards are crucial to winning -- the team with more map awareness is the team that can take advantage of opportunities.