If you want your muscles to grow and become strong, you need to overload them with progressively heavier weights. This is called progressive overload.
Take a little time to find the right weights for your exercise. Let's say you're doing a barbell bench press. Begin with the bar all by itself. Press it up and down. Is it easy? Then take two 10-pound weights and place one on each end of the bar. Again, press the weight up and down. Still easy? Try adding a 2.5 pound plate on each side. Continue this process of adding weight until the exercise becomes challenging to complete in, say, 8-10 reps.
Don't forget to perform the exercise correctly. Never sacrifice proper form for the sale of lifting heavier. Making sure your muscles work hard with proper form is what progressive overload is all about.