How do I know how much weight to lift if I’m just beginning strength training?

The first step to beginning a muscular-fitness training program is to determine what your “maximum strength” is. Most strength-training programs are designed around lifting a certain percentage of your maximum strength depending on your fitness level and your exercise goals. The most common way to assess your maximum strength is the one-repetition maximum test (1RM), the most weight you can lift once during an exercise but not twice. The second step is to determine what percentage of your 1RM—that is, the actual weight—you should use to improve your muscular strength. Typically, you can improve your muscular strength when you use 60–90% of your 1RM. As you get stronger, you should periodically reassess your 1RM and adjust your weight (that is, to 60–90% of your new 1RM) accordingly.

References

American College of Sports Medicine. (2017). ACSM's Guidelines for Exercise Testing and Prescription (10th ed.). Philadelphia, PA: LWW.

Reynolds, J. M., Gordon, T. J., & Robergs, R. A. (2006). Prediction of one repetition maximum strength from multiple repetition maximum testing and anthropometry. Journal of Strength and Conditioning Research,, 20(3), 584–592. doi:10.1519/00124278-200608000-00020