If you are a beginner, I would recommend that you get at least six solid months of training in before you try to perform forced reps or negatives. This is mostly due to safety, as your tendons will not be used to the force placed on them using these techniques.
Forced reps are usually done with a spotter, however some like bicep curl can be done without one. When you reach failure during your last rep for the set, let’s say during bench press, you can bring the weight back down to your chest and your spotter can help you bring the weight back up so that it is a little lighter and you can work your muscle further.
Negative reps have a similar goal in mind, to work the muscle further. The difference is that your spotter brings the weight all the way back to the top of the exercise and then allows you to slowly bring the weight back to your chest. The logic behind this is that since muscle growth is only stimulated by the eccentric part of the lift, the part where you lower the weight back down during a bench press, forcing a couple of those at the end of a set will stimulate further growth.
These techniques can be useful for anyone trying to build mass and has hit a plateau or trying to break down their muscle fibers further. These are great to use during power workouts or times when you are training with low volume or frequency, as these techniques might contribute to overreaching or overtraining if done in high volume workouts. I also would not recommend to perform these ofter or at all on a diet (caloric deficit) because when you are on a diet, it takes longer for your muscle to recover and these techniques may cause overtraining or overreaching.