After gathering some information, you want to create a list of the fish you would like to put in your aquarium. You should identify the order in which you will introduce each fish. My biggest suggestion to you, no matter how hard it might be, is to not deviate from the list and purchase a different fish unless you have performed additional research. You want to make sure the fish you purchase will be a good substitute, and still compatible with the other livestock you carry in your aquarium.
The next step for you in this process is to actually complete the list of fish to purchase. This list is highly dependent on the owner of the aquarium, and should be created by you to make sure you enjoy what is in your aquarium. The suggestions I would provide when creating this list is to carefully consider the size of the fish, their temperament, reef compatibility (will they eat corals), and ideal aquarium size for their species. You want to make sure you purchase a fish that will grow to a size that is sustainable for your aquarium tank size. Ensure each fish you purchase will get along with the others in your aquarium ecosystem. Some fish species are not as compatible with others, and you want to make sure you identify this in your research. The order in which you introduce the fish can be very helpful if you are intending to purchase a highly territorial fish. Lastly, nothing is worse than purchasing a fish to realize they just ate through your entire new coral overnight. Make sure you purchase a fish that is reef-compatible, if you pursue a reef aquarium. Hopefully these tips will help you get off to a great start to your journey!
I hope these tips provided you a good starting point for helping you identify the fish to purchase.