So, I’m a little into nightmare on my Barb and have noticed my damage is getting to be way too low.
What bosses should I farm, and for what gear? I need to get myself to a point where I can solo Baal on nightmare and I’m not sure how to go about it
Have you tried upgrading your current gear? Normal->Exceptional->Elite? (Don’t upgrade to “elite” unless you are well into Level mid-70’s.)
I use War Axes, I used my rare(yellow) items to upgrade them (to Naga’s) using gems and runes in the cube. If you can find a decent 3-socket chest armor, you can make ‘Myth’ which adds +2 to barbarian skills as well as missile defense.
I upgraded my best Naga to Berserker Axe and now I can’t use it until im lvl 76 
And I just barely reached lvl67
What Build are you using? that may help us give more specific info.
Upgrading gear always helps.
Look at the lower tier runewords, there may be one that can help you break past that threshold.
Also, when it comes to nightmare, don’t be too eager to jump into Hell, especially with a melee character. Best places to farm would be Mepisto, chaos sanctuary (kill diablo if it’s easy enough for you), and in act 5 go for the two bosses north and south of the first waypoint, and pindleskin.
At your point I’d be looking for runes and Pgems to trade with as well for specific gear.