Hey, I've played quite a bit of Cassiopeia on my smurf, which is currently mid plat.
Firstly, I would advise playing Cass mid only. She can be played top but she only has a few good matchups, most popular picks destroy her and she's not really what you want from a top.
Masteries: Go 21/0/9, it offers way more than 9 in defence (only the HP is good in defence). The mana/health regen masteries synergise really well with Tear/RoA and either biscuits or runic affinity are great for Cass, it's all your preference.
Runes: Magic Pen marks always, Scaling Health or Armour seals, Scaling AP or MR glyphs and AP or MS quints. All of this is preference or matchups based, but never run CDR in runes - Sorcery and your 250 stack passive provides 30% CDR then blue buff handles the rest.
Summoners: Always flash, teleport mid is the current meta - gives you a safe lane and great roaming + late game power (likely to change once S6 rolls around), cleanse can be great vs Annie/Liss/Veigar/etc, exhaust for assasins with a long-ish combo that you can outplay (think Zed), barrier for bursters like LeBlanc, ignite is still fine in most matchups, ghost is also decent if you like it.
Itemization: Always Tear or RoA first. Never Athene's (only get Morello if you NEED grievous wounds) because you'll go past the CDR cap and it doesn't give you any mana - a large mana pool is essential later on for E spam. Archangel's is preferable to RoA as it gives more AP and mana as well as some regen but RoA is good for hard matchups. You probably won't want to buy both items because it takes too long to scale up and leaves you with very little damage until both items are finished. You can also do 2x Doran's Ring for a stronger lane.
Aside from that, you can build most AP items on Cass. Abyssal is decent for hard AP matchups/AP assassins, Rylai's is amazing for kiting + making you a bit more durable, WotA is situationally good if the enemy team has no burst. Zhonya's and Luden's are both good. Liandry's is less good on her now than it was pre-rework as her damage is all on her E now, not poisons; it's still perfectly fine if the enemy team is stacking HP though. Make sure you always incorporate Deathcap, Voidstaff and Sorc shoes (really good with Furor enchantment if you don't need distortion/homeguard) at some point.
It's a bit harder to first blood your enemy level two these days, but it's still possible; you need to land a lot of Q poke level 1. Due to the fact that your passive doesn't reduce mana costs anymore, it's very easy to oom before you can get the kill. Generally while you're laning just land as many Qs as possible for poke and passive stacking and follow up with Twin Fangs if safe. Expect ganks + ward accordingly as Cass is really vulnerable.
Last-hitting is easier now: poison minions and last hit them with your E to refund the cost, you can sometimes just last hit with E alone for 1.5% mana pool restore without incurring the mana cost of casting a Q, if you can spare the cooldown. Pushing is harder now though as your poisons barely deal any damage. Roaming isn't anything to write home about, however running Teleport Mid is meta right now, so that makes your roaming 100 times better.
Teamfighting is so different from game to game. If you see a pick opportunity you can Ult Flash (Cast your ult and flash forward during the animation - much easier to catch people off guard, your ult goes in the direction of your flash so you can even ult behind yourself and flash forwards and it will still land) on priority targets. Most of the time you'll want to just hold your ult for self defence unless you see an opening to hit their carries. I usually play similar to an ADC in that I'll hit my poisons on the closest target and go to town with E spam until it's safe to go for more important targets. The key is just to be really cognizant of your surroundings in teamfights and make sure that you land your poisons, your damage will be nonexistent if you're unable to spam E.
As a side note, just realize that Cass is an incredibly binary champion as she stands currently mainly because ALL of her power lies in her E, previously she could at least deal poke damage with Q, not anymore due to its terrible scaling. If you get behind you will probably stay behind unless you're able to make a play and it's very difficult to come back from a deficit because in order to E enemy champs you put yourself in danger. Cass can be really frustrating but if you're devoted to learning her then go for it. If you want to watch a master/challenger level Cass play, I would advise watching http://www.twitch.tv/lol_king_cobra when he streams. He doesn't seem to stream very often these days but it's definitely worth watching when he does.
