Season 7 ends in November. In your situation, I would continue using tier 2 runes. Tier 3 runes are expensive and it could take awhile to complete a full rune page. Before playing ranked, I recommend continue playing normal games passed level 30. Since you're new to the game, focus on what type of champions you want to play.
(Focus on buying champions you are most interested in and want to main. Since Riot is creating a new "Runes Reforged" for pre-season 8, runes will be obsolete.)
Once you find your favorite champion, practice and understand that champion inside and out. Learn the mechanics and positioning.
If you're comfortable with the champion and you're willing to buy tier 3 runes, go for it. You'll most likely have saved up enough IP from all the normal games played passed 30 when practicing. At that point, you might be ready for Ranked.