Failure is simple, it's the very last rep you could do with that amount of weight. Most people, at least for the first few weeks, will need to drop weight on occasion between sets, but it's amazing how quickly your body adapts using that method. Although I should add the proviso that I don't do traditional bb bodypart splits. I divide into push/pull and bi/uni lateral movements. I do one olympic lift to start and after that each type of movement only gets one exercise per session. And since OP said he wanted overall fitness this is what I'd recommend, if you're doing traditional bb program where you are doing 4 or 5 sets for a bodypart in a given day exhaustion on each set would probably be too much, so I'd say go a load where you could probably get one, maybe two, reps out on each set and to exhaustion on last one.