[D&D5E] Small Question on Bonus Damage

I have a weapon that does 1d8 slashing and 1d6 fire damage, and another thing that gives me +2 to my damage rolls. My question is, where do I apply that +2? Rather, would that +2 go to my Slashing damage, or my Fire damage, or would it be +2 for both?

The weapon description should tell you. If this is a homebrew item, then you should ask your GM. Normally, a weapons damage bonus applies to its base damage only unless the item description says otherwise.

What is the ability that gives you +2 to damage rolls? 

In this case, it is Hexblade Curse, providing damage equal to my PB (2 in this case)

Generally the interpretation is to select the type of damage you want to add the +2 to. The basis for that is 1d8 slashing + 1d6 fire is one roll.   So in this case it could be +2 slashing or +1d6 fire, your choice.  However, if you are attacking with something that provides multiple separate damage rolls (such as Eldritch Blast) then it applies +2 to each damage roll. Ultimately it is up to DM interpretation.  Note: This is off-topic for Roll20. I would consider asking D&D related questions on the D&D Discord server .