there are 2 key ways of using necrons, the lots of warriors route, and the minimum warriors route. I generally go for the latter as i like to have some variety in my list where possible.
When it comes to your units, i find taking 2 of each of any that are going to be important to your game plans is a good idea, that way they can stick together to ensure those wbb roles etc.
The monolith is a great addition to the army, mainly for all the things it does for your army. Re-roll wbb, teleport into rapidfire range, teleport away from cc, disrupting the enemy. It makes the humble warrior into possibly the best troops choice in the game.
immortals and destroyers are both great units, so most armies can't go wrong by starting of with a couple of units of them for mobile firepower. I think wraiths are really good when used properly, and have often had them wins games for me against opps that dont rate them. You just have to make sure there not facing down lots of small arms firepower. Scarabs can forfill the same cc role, but at the opposite extreme. They can handle the boltgun shots, but can suffer vs lots of str6+ weapons fire.
The list i use at the moment which i think is quite balanced and gives a nice mix whilst still being able to handle most lists at 1500pts is
Necron lord on destroyer body, warscythe, res orb[goes with wraiths]
10 Necron warriors
10 Necron warriors
4 destroyers
4 destroyers
3 wraiths
2 hvy destroyers
1 hvy destroyer
Monolith