2d6+modifier to beat target number help

Hey everyone. When it comes to testing player character abilities, I like the bell curve of 2d6 and also want ability modifiers to play a roll in adding to 2d6 rolls under specific circumstances. Though these modifiers will be low, with +2 being the highest you can get at level 1 and only for one ability. The highest they get is +5, and only one ability would ever be able to get that high. Something else I like is player facing rolls and target numbers. Players roll to attack and try to beat their opponents defense score. The player is attacked and tries to beat their opponents attack score. What I’m not sure about is what the target values for varying difficulties should be. I would appreciate some advice and opinions.