Summoner spell substitution: change heal for barrier, exhaust, cleanse.

Runes, masteries, items highly depend of playstyle and team compositions.

You will struggle with high burst and hard CC champions. Teemo is not a threat since you are a caster adc. Most likely trouble with tanky/cc/maybe high burst supports on enemy side.

Not much to talk about here, when you see the opportunity just throw everything you got at enemies. Don't spam mana for no reason. And use your ult until you get a big rocket, then save it for engage. And don't go rambo with passive from the base, you may underestimate your and your enemies damage output.