Never work to failure. Your muscles will get re-used and might actually fail.
Have daily goals. If you are learning some exercise, get the form right and do just a couple reps with perfect form. Then rest for some time and try a couple more. Continue until you reach your goal or your form degrades. After a few sessions of this, you will find it much easier to hit your goal. Then try for three in a row. Continue adding a rep ever so often until you can do 10 reps. More than 10 reps means you are working on endurance, i.e., training the muscle to recover so it can work again at the same load (physiologically you are growing more mitochondria). This might just eat up your practice time without much in the way of payback. Seriously, 10 handstand pushups are great, but there is something wrong with your martial art if you need to have enough endurance to do 50 of those in a row.
Oh, sometimes people ask me about using medicine balls for training. These are really good, but don't go get a big one. Something in the range of 2 - 3 kg (so about 10 lbs. is the absolute top limit) is good. You want to get one specifically designed for throwing and play catch with various tosses off a concrete wall. This lets you really develop speed, power and the ability to receive and impact. Folks usally get a really heavy one, which means you can't move it fast enough to get much benefit, or they toss them back and forth to each other, which you have to do pretty slowly or you'll blast the other person of their feet and probably injure them if their timing is off. In short, sure, get one and use it if you like, but do it right.
Each exercise has several sections. Here is what they mean