Early you will have no real damage or skills for attacking the enemy in your lane. force your opponent to push to your turret. Using your armor and potions you can (should) farm until level 8-9 before the first basing. use your Q to farm it only on the caster minions for the first 15 or so stacks, then (or whenever you are ready) you can start to Q melee minions. (You should always Q the siege minion) (66 Q farm = 200 extra damage)

After you hit 6 you should have decent farm. try to trick the opponent (if you believe you can) into coming close to you, use E > W > AA > Q to damage him near your turret, then back off. After this when the cooldowns have finished, break out and kill your opponent. Using ult is situational, however due to the low CD you should use it to make sure. Now the tricky part about this, if you use your Ult before you go in, the enemy might suspect a gank or know you are going in for the kill. Try to engage and bait using your ult, when the enemy becomes to committed, activate it.

If you kill the enemy and have a wave with you, you should be able to destroy the turret with about 10 or so minions depending on your Q farm, I find this is almost guaranteed at 6 (and a siege) minions at about 70 Q farm.

Depending on how you do in your lane you can continue farming minions for your Q (what i do until around 25 minutes or at least 300 Q farm) the game should then be easy as with the items suggested, not many people can kill you on their own, I have found that even in losing matches, as soon as the farmed Nasus comes down to FINALLY help his team, they win.

Tell me how it went.