Well as you probably know Veigar is not a very popular champ but it's very effective in teamfights, it's fragile and squishy but if you're well positioned, your basic combo is to hit enemies with your E to stun them, follow that with a W, and then you hit your Q and when the electrocute hits follow with your R that deals damage on missing health.
You have to play safe in the laning phase, veigar is very vulnerable to ganks since he has no escape but his flash, you have to stack as much as you can and try to carry 1v9 which is not impossible.
A simple tip against assassins and champs that jump into you, just put your stun on yourself, so when they dash to you, they will be stunned and you'll follow with the basic combo.