RE: Nasus. Like I said, wait to use W until you need to escape someone (they are in your face and you don't want to fight them because they deal more damage than you) and E when you need to clear the enemy minion wave quickly. Use both together to maximize damage when you go in for the kill. Don't go in for the kill until you're sure you can out-damage the enemy, which generally happens when you have 200+ bonus damage on your Q and get a Sheen.
My advice for building Warwick focused on CDR. I assumed that's why you bought Morellomonicon in the game I saw on LoLKing where you built that on him. Nashor's Tooth actually isn't a grat item on Warwick, but it's a better item than Morellonomicon because it plays to his strengths better. You obviously wanted a damage item that gave CDR, and for Warwick Nashor's Tooth is a better fit for that niche than Morellonomicon is. You were also many, many kills ahead so building tank didn't make sense.
In general, I would build a Spirit Visage to give Warwick CDR because it also provides good magic resist (you usually need at least one MR item) and it boosts his healing and lifesteal by 20% with its unique passive. Warwick is a "drain tank" so he soaks up a lot of damage by healing while he deals sustained damage over time to one target. Anything that increases his sustained DPS (attack speed, attack damage, on-hit damage) is good, as is anything that increases his ability to heal.
A full Warwick build would be something like Enchantment: Devourer (or Warrior), Blade of the Ruined King, Wit's End, Spirit Visage, Frozen Heart, and your choice of boots. That provides good damage, good attack speed, extra lifesteal, full or near-full CDR, and some tankiness so you don't die to burst damage before you can hurt people. it also gives you the Blade of the Ruined King active ability to slow enemies so you can chase or escape them easier. There are probably better builds on Mobafire or wherever, but that's a quick example of picking items that suit what the champion needs to do.
League of Legends is a complicated game. Some of the advice you'll receive is "conflicted" because it's situational. For example, there is no one true item build for each champion that must be followed perfectly because every game is different. There are items that are generally good on certain champions because they play to those champion's strengths. There are items that are sometimes good on certain champions depending on the enemy team. There are items that you will pretty much never buy on some champions because they just don't provide useful stats. When you plan to play a champion, take a moment to think about what that champion does for the team and which stats they need to do it. Build items that give those stats.
You seem to be getting overwhelmed by all the options right now (with all the advice you;re getting, not itemization in particular). I would suggest taking one champion you like and practising with them for awhile. Go to Mobafire and look up a build guide for them. It will give you a starting point for what kind of items you should build on that champion and explain why. Feel free to experiment, but at least compare the items more experienced players recommend to the items you usually build. The rest is just practice. You will get better at both mechanical things like last-hitting and dodging skillshots and strategic decisions like when to go for dragon and when to push into their base with time. You'll get a better feel for who is and isn't a threat to you each game with time.
There's a reason you need 16 champions to play ranked. In addition to the rare possibility of not having enough after everyone else has picked and banned, making sure you have more champions ensures you take a little more time to learn before jumping into ranked. That's all there is to it. Practice, practice, practice.
