As a beginner, your goal shouldn't be performing your exercises to failure. More experienced lifters do perform exercises to failure as their final set but as a beginner, your body needs to adjust to this type of strain. That being said, your main goal should be completing the workout feeling challenged but not too overworked.
During your workout, if you feel like you can do more, please consider some of the following:
- Add an additional exercise that targets the same muscle group
- Add an additional set to an existing exercise
- Increase the number of reps in an existing exercise
- Increase the amount of weight
When making any of these adjustments to your workout, please keep in mind that your exercises should be challenging but DOABLE (especially during the final few reps of a set) with good form!