QuestionHow do you know what size jock strap and cup to buy?
Jock straps are sold by waist size. The easiest way to figure out what size you need is to look on the tag of a pair of undershorts that fit you well and buy a strap that is the same size. If you want the best fit, see if your mother has a cloth or vinyl tape measurer -- your dad's steel tape measurer will not do in this case. Take off your clothes and find the top of your hip bone, wrap the tape around your waist, just below the top of your hip bone. Overlap the end of the tape against your skin, but don't pull it tight. Read the size at the end of the tape measurer. In the United States men's clothing is sold in sizes based on inches, so if the tape measurer read 27 5/8 inches, then you need a strap for a 28 inch waist (always move up to the next highest whole inch). Since straps have elastic waist bands, they are measured in ranges, such as 27-30 inch waists. Since you are probably still growing, avoid getting a strap where your waist is at the top end of the range. For example, if you have a 28 inch waist and your choice is a 26-28 strap or a 28-30 strap, get the latter one as it has more give and will last you a while during your growth.
At least in the United States, I've notice that cup sizes are always called large or extra large, no matter how big they are. Men refuse to think of their genitals as being medium or small for some reason. Often a cup will come with a jock strap, but if not, you will probably have to guess at the correct size -- there is no consistent measurement standard for cups. What you are looking for is one that will contain your penis and testicles, but without being so large that the edges of the cup ride on your thighs. Measure the width of your groin between your thighs while sitting down with your legs spread slightly apart. Take a tape measurer with you to the store if you can and find a cup that doesn't exceed that width. Often times though, I have found the cups in many stores to be basically all the same size, no matter how they are labeled. Getting a cup that is too big will cause your inside thighs to get irritated from the cup rubbing on them, so when in doubt, go for the smaller size. Too small of a cup carries the risk of the edge pushing on a testicle when you are hit in the groin -- the very thing you wear a cup to avoid.