Generally you should start to fail at 8 - 10 reps and if a spotter can help u push out an extra two while you fail your reps then that is excellent. 3 sets of each exercise.
Ok brother. Firstly, think about what exercises you're doing. You may have been doing the same thing for too long and need to change your routine. So you've got different parts of the pectoral that all need attention (upper, lower and middle). But you also have two types of exercises, pushing for width, and fly type exercises for wideness of the pecs that really shape them.
You should do all of these exercises in your routine except for bottom pecs where you can alternate between dips and cross overs. Of course in your routine you can vary how you do your exercises between your routines but that's at your discretion.
Upper. Like I said two types of exercises and both are imperative.
Incline bench press x 3. Incline bench is usually tougher than normal because its a weaker part of your chest so you'll find you're doing half or two thirds of the weight you bench unless you have quite strong shoulders. You can mix it up by doing dumbell press or barbell press. You'd be surprised by how much of a difference it can make by switching between the two when you need to change your routine.
Incline fly x 3. Same principals here but with fly. You can use cables or free weight.
That's upper for me. Switch around between free weight, cables and bars if you feel your body needs a change.
Middle. I won't repeat myself too much.
Bench press x 3. Switch between bar and free weight! But free weight should be priority. It will give you a nice stretch in your pecs and it seems to be generally more effective.
Fly x 3. Same thing. Cables and free weights. I use cables standing mostly. Its hard to get free weight right with form.
Note: If you really want to destroy your pecs. Between each of these sets do either 6 more reps of a lighter weight or push ups just to push them a little bit extra.
Bottom.
Declined bench press x 3. Exact opposite to inclined of course. Adjust your bench so you are on a downward angle and bench like normal. Only free weight is possible with this unless you can find a declined bar bench

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Dips x 3. If you don't know it. Youtube it. For extra weight you may need a weight belt or put a dumbell between you feet. You may do up to 15 reps with dips.
Cable cross overs x 3. My favorite pec shaping exercise and finisher! Not much to say. Pretty lightweight exercise i allow myself up to 15 reps again with this.
Note: always do the declined bench but you may alternate between cable cross overs and dips. And if you didn't do push ups between your middle pec sets then do them as a finisher after your dips/cable cross overs.
I don't think i'm forgetting anything. Remember to change your routine when need (for me about every month or so). Good luck buddy.