You really need to send pictures. Think about being in our place If this is an old leaf, no big deal. What is the lighting? What kind of pot do you have this plant in? Avocados need lots of light, some humidity (boil water on top of your stove, simmer all day and night to keep humidity between 40 and 60%), use Osmocote for fertilizer, make sure your pot isn't too huge for the size of plant, good, sterilized potting soil, no rocks or gravel beneath the soil, a drainage hole, keep the saucer below free of water, get mycorrhizae and bacteria for your soil if they weren't included in the soil or fertilizer you used and when you water, soak the soil and then allow to dry out somewhat between waterings. Put the whole plant in the shower to water and drain...once a week. Don't overwater!
Send pictures so we can see what is really going on. Indoor plants suffer from too little light, too little humidity and too much water. Especially tap water high in salts. Lower leafs get old and it is normal to have them die off. Without being able to see your plant and know more details, this is the best we can do...Send pictures and more information. We can help.