Indoor plants are planted in potting soil and over time the nutrients in that potting soil are lost over time. For this reason a periodic change is potting soil is a good idea, but generally that is not something that is done frequently. You can supplement those nutrients with fertilizer, but be aware that not all plants need fertilization. Generally speaking plans that are considered low-light plants don’t really need fertilized. It is much more damaging to the plant to over-fertilize the plant, than not to do it at all.
If you do decide to fertilize your plants choice a product with a nitrogen (N), phosphorus (P), potassium (K) ration of 15-30-15. You may notice that the fertilizer has other important nutrients too like iron, magnesium, copper, zinc, and calcium. Fertilizers come can come in the form of pellets, powders, or liquid. Any of those forms is just fine, but one thing you might want to pay attention to is if the fertilizer is organic or chemical.
Most people would say that the best time to fertilize plants is in the spring for outdoor plants, since that is their growth season. Most plants, including indoor plants only need to be fertilized once a year, maybe twice. What you don’t want to do is over-fertilize, if you are concerned about that then it is a good idea to dilute your fertilizer in water.