When a piece of clothing does not fit, you can have it altered. But this is not always worth it. If the clothing has to be heavily altered, it may cost less to just replace it. However, that does not mean alterations, in general, are not a good idea. There are several alterations that are easy and inexpensive to have made and that are almost always worth it.
Shortening skirt or dress hems.
You find a skirt or dress you love, but the hemline is just a little long for you. This is very common. In fact, the higher-end dress and skirt makers just make their products longer and assume some customers will have them hemmed shorter. This way, they can fit people of all heights with one product. Shortening a hemline is a very easy task for most tailors. They just have to cut the material, fold it over, and sew a new hem. This will likely take a short amount of time, so the fee won't be large, and it may be the only way to get a skirt with a suitable hemline if you're on the shorter side!
Taking in shoulder straps.
If you purchase a dress or top without sleeves, the shoulder straps may be a little long for you. This can cause the front of the top to hang too low. Shortening the straps is very simple. Your tailor can remove the stitches that attach the strap to the top, cut the strap shoulder, and then sew the strap back into place.
Slimming the waistline.
If you have a fitted shirt or even a dress that seems to have too much material around the midsection, you may figure that it's just the wrong cut for you. But actually, a tailor can fix this more easily than you'd assume. They can stitch some triangular "darts" into the fabric right at the waist to bring the waistline in. The best part is that if you gain weight in the future and need the waistline let back out, they can just take the darts out or shorten them, restoring the piece to its original cut.
If an article of clothing does not fit you in the slightest, by all means, leave it on the rack. But if it only needs to be shortened, to have the shoulders taken up, or to have the waistline slimmed, it is worth finding a tailor who can make these alterations.
To learn more, contact a tailor that does alterations for clothing.