My best advice to you to level up to 20 quick is to just go through the story mode. After each mission, return to the tower to check for bounties. Do all the story mode strikes along the way as well. Once you hit level 20, buy the Future War Cult bond for your Warlock, and level them up. Wear that bond when you complete strikes and turn in bounties. Future War Cult has a chest armor that increases the amount of ammo you can carry for hand cannons AND sniper rifles... I know because it's the exact armor I use! You will need to level up with FWC to level 3 to get that armor though (because you will need a Crucible commendation in order to purchase it).

Adding on to what he said. For handcannons in PvP, reload speed is a must unless you have a high magazine size. Handcannons don't fire too quickly, but if you can learn to slightly adjust your aim between your shots you'll be able to hit 90% of the time wherever you want. I wouldn't recommend using them from long range either. Close- mid is the way to go and hip firing isn't something you should do unless you are at close range. Good perks to have would be things like Increased reload speed when the weapon is empty or increased stability. There are many more but for someone who wants to main a handcannon those work well in all situations. If you play as a Sunsinger I wholly recommend you go after Thorn. If you see an Exotic bounty titled "A Light in the Dark" you should take it. The burn damage of thorn does stack with sunsinger ability or grenade burn and it is a devastating combination. If you need help with effective setups for PvE or PvP feel free to let me know.

Awesome, if you have an Xbox one and need an extra person or help, I'm a 31 on every class. My GT is REAPERxJSATx