Early game as you may already know is awful for Nasus, but not so bad if you play competently. First things first, never EVER randomly through out an E or W at the enemy unless you can kill them, or to get them off of you. You will go oom and push the wave to their tower, which is a big no-no for Nasus. You want to last hit Q minions while keeping yourself safe. Are you losing half your hp while going for a creep? If you are, then you have to play safer. You want the wave to push towards your tower, but just outside of the towers shot range. You can take some minion aggro so that you can freeze it and heal up with your passive. If you are forced to farm under tower, remember the amount of hits each minion can take before dying. Just make sure not to die! Even though you scale into the lategame, your teammates won't want to deal with a fed Jax oneshotting everything.
I don't know how good you are as Nasus, but here is a general rundown for how much stacks you should have. 10 minutes: 100 stacks (this is if you are doing not bad but not good) and by 20 minutes: 300-400 stacks. For ruins, you want some flat cdr quints and glyphs. Mix that with 5% cdr from masteries and you got 20% cdr. For Nasus, you want to hit 40% cdr ASAP! That way, your Q only has a 2.4 cd, getting you stacks galore. With the runes and masteries I mentioned, you hit 40% cdr with only one item, frozen heart. You do not want to leave your lane almost ever. Unless your team REALLY needs you, then most of the time you will stay in your lane until you get maybe two or three towers down. Nasus is one of the best splitpushers in the game, but you don't want to get ganked by their whole team. Make sure to have where you're splitting warded well. After a certain point, you will feel like god of death MURDERING EVERYTHING IN YOU'RE PATH (unless they have a fed vayne, then GG).
And yeah, that is Nasus really. Here is a guide by Trick2G on playing Nasus. Just remember to relax and have fun. And default Nasus is the best skin.