If your looking to improve your appearance and your performances, then you need to continuously make greater demands on your muscles when training.
Your body will not change unless it's forced to, outside of that comfort zone.
Your body is smart, muscles will always adapt to the stimulus you give it, so if your just doing the exact same things over and over, your muscles have no reason to make further adaptions, and this is where you plateau and no longer see results.
So, in order to continually make muscle size and strength gains, you need to PROGRESSIVELY OVERLOAD your muscles.
How can you apply this principle to your training then?
There are a few different ways you can increase the demands and progress over time...
The main one is to lift heavier weights. By increasing the resistance you use, your muscles will have to adapt to be able to handle more loads, and will increase in size and become stronger. You won't be able to increase weights every single week though, and should not come at a cost of your exercise form...
Which brings me to another way to progress weekly in the gym, simply by bettering your technique. For example, going from being rounded over in a deadlift, to being able to maintain a neutral spine and smoothly push the hips back. This will help you to progress that exercise, become stronger at it and develop the target muscle better.
You can also increase the number of reps you do with the same weight on a given exercise. Say for example you did 30kg x 10 reps on squats one week, then managed to do 30kg x 11 reps the second week, that's progression. With this however, you wouldn't want to keep on increasing the reps higher and higher all the time, if you had a rep range of say 8-12, and you was hitting 12 or above, then it would be better to focus on increasing the weight and aiming for the 8 reps, and then can progress the reps again from there with the new higher weight.
Increasing the volume is another form of progression. Volume is the number of sets x reps x weight used. So if you increased the number of sets in your workout, that would also increase the demands on the muscle. So for example doing 4 sets of 8 reps on an exercise instead of 3 sets of 8 reps. Again, you wouldn't keep on increasing this, as too much volume in a workout can take it's toll on your ability to recover and on ability to progress through other means.
You can also decrease the rest you have between sets. If you manage to lift the same weight for the same number of reps for each sets with shorter rest periods, then that's a progression on your performance. So for example, having 60s rest instead of 90s. You would need to time your rest period's properly for this.
Another little way you could make progression is by increasing the Time Under Tension with the same weight, this is, how long a muscle is under constant tension during the set. So for example, if one week you were doing a chest press with 20kg for 10 reps but were rushing through it, dropping the weight down too quickly, bouncing out the bottom to push back up and completing the set in 15s, then the next week you slowed down the eccentric (lowering phase of the exercise), counting 3 seconds on the way down, paused for a second at the bottom, then smoothly pushed through, completing the set this time in 40s, then again this is increasing the demands on the muscle to grow and improving your performance.
So ensure you are applying the principle of Progressive Overload to your training to make progress over time, using one or more of the above means.