It's very common for new cat owners to make mistakes in caring for their cat. It's doesn't mean that you are a bad cat parent. It just means that some more research and education is necessary. The most common mistakes are usually around food choices, litter box training and maintenance and healthcare.
Poor Food Choices
There are many myths about cat food and the nutritional needs of cats. Some cat food companies create products that look like they are healthy because they are "recommended by vets". They also sell their food by stating that the food is engineered specifically for the cat's health. Nature is the engineer you want to go with. If food is so low in nutrition that a scientist has to add nutrition back into it, it's not worth eating.
There are also myths about kibble. While kibble helps to remove plaque from the teeth, it should not be the primary source of food. What keeps a cat's teeth healthy is proper dental care including teeth brushing and teeth cleaning at the vet's office. Dry treats or tartar control treats are another part of dental hygiene. The nutrition in kibble is not substantial enough for a cat's daily needs. Even a high quality dry food doesn't give your cat the nutrition she needs. Moreover, cats need moisture they can ingest through the intestine in addition to fresh water.
Choose a natural cat food with whole ingredients and plenty of moisture. A raw food diet is best but a high quality canned food is a good second choice. Look for a formula that focuses on whole, lean protein and one that is low in grains, fillers, and minerals. Too much mineral content can lead to serious urinary problems in cats.
Litter Box Mistakes
Many new cat owners don't know how to train a cat and miss the mark on litter box maintenance. Even an outdoor cat needs an indoor litter box. Cats can be trained to use the litter box. It's unlikely that a cat will use a toilet and flush it but you may be able to train her to sit on the toilet. The biggest mistake new cat owners make, however, is not cleaning the box frequently enough or making mistakes with litter choices. A cat's box needs to be cleaned out at least every other day but some cats will insist on every day. Try to avoid switching litter but if you need to, do it over time. Mix it with the old litter for awhile and see if your cat likes the new brand.
Healthcare Mistakes
Many new cat owners neglect to get health insurance and vaccinations for their cat. If you wait until your cat is sick to get health insurance, you most likely won't get coverage for the illness they have. Avoiding vaccinations can lead to serious disease, and having insurance helps lower those costs.