1 - Choose a starting weight & rep range for an exercise.
2 - Every time you perform that exercise in the gym, aim to increase the reps you can do with your starting weight until you hit the top of the rep range.
3 - Increase the weight you're using (a 5-10% increase is usually okay).
4 - Go back to step 2 and repeat this process.
If you constantly repeat this process with at least 1 or 2 exercises in each workout you'll see great results in the gym.
Drop me a message if you have any questions about this.