you should get a pair of briefs or boxers that are specifically made for packing with (or DIY one if you're into that - there are instructions for how to do that floating around on the internet). in my experience packing without any kind of harness is kind of cumbersome, but the packing briefs are good.
I have always just used the velcro top ftm pouch from The Transitional Male. that way I can wear whatever underwear I want and my business stays put.
In the interest of full disclosure I'm an underwear enthusiast/snob. I don't care for the idea of being limited to just the "packing" underwear currently available. Not to mention, If I'm going to pay that much per pair they had better be a lot sexier than plain black generic looking underwear.
What's best for you depends on how much money you have to spend, but some sort of harness definitely helps keep your packer more secure. I generally use either a SpareParts packing strap or else wear really tight boxer briefs that hold my packer relatively securely on their own. You can also make DIY harnesses - there are a lot of guides online if you google the phrase - or even just tuck your packer into a sock and safety pin the sock to the inside of your underwear, if you need more security in a hurry.
A packer is what some trans* guys and genderqueer folks tuck into their underwear to create a bulge and help them feel happier/comfier/sexier /whatever they'd like to be feeling in relation to their bodies. There's a whole page for packers on this very website: link
It depends on the packer, the underwear/pants you're wearing, and your personal preference. I've found that depending on the jeans I'm wearing, my packer will work fine tucked into the flap of ordinary briefs or boxer briefs, but more often I'll keep it in a homemade harness just in case.