I have 650 normal wins total and over 4000 ranked wins (at least) so take it from me, you can and should start playing ranked the instant you hit 30 and should try to buy 16 champions by the time you hit 30 so that you can do so. The reason for this is simple: playing normals is an utter waste Of time. You will not improve much (and will get worse after a certain point) and won't be playing for anything.
Read guides (don't watch streams) to learn what champions do and how they interact. Most importantly though, learn from your mistakes. If you minimize your own deaths that is 90% of the battle. The situations in which it is acceptable to follow a teammate into death are few and you won't be able to distinguish them for a long time, so always err on the side of caution.
Always ignore your teammates (though do not put them on ignore, simply don't respond if they address you) in solo queue. They will rarely be constructive via chat and even less often will your responding be constructive, so just don't do it.
Never spend Rp on anything except for skins.
Buy general purpose runes first and don't worry too much about rune page setups - specialized setups aren't generally worth it until well into Diamond. Just take the obvious stuff and focus on your own play.
Finally, learn how to play one champion comfortably in every role but particularly support. For whatever reason terrible league of legends players (aka the ones you'll be matched with as you learn) simply can't play support because they carry so hard, so you'll find yourself playing it a disproportionate amount of the time.
Finally, never get discouraged at a loss. Even he best players lose well over 40% of the games they play and most lose as often (or more) than they win. Learn what you can from one game and then leave it in the past. Never dwell.
Best of luck!