Fish for poke with Q and if you hit it, you can combo (EWR or sometimes just R) off the slow. You don't need to play really risky on Vel'Koz since his poke ability is really long range. It's usually better to kite back and let opponents run into your abilities.
If you can't make much happen in the early game that's okay. He's quite strong in team fights, so you should do fine even if you're just farming in the laning phase.
Playstyle depends completely on your matchup. Typically you want to play very safe and just shove your lane against assassins, and you can play aggressive against easier opponents. For more detail look at the matchup section of any good Velkoz guide.
If you can't make much happen in the early game that's okay. He's quite strong in team fights, so you should do fine even if you're just farming in the laning phase.