Start with a larger tank so you have more options and more cushion if something goes wrong.
And research, research, research. That simply cannot be emphasized enough. Don't ever buy livestock unless you know how to care for it. And no, advice from the store doesn't count unless you've established that they're trustworthy. Many stores will try to sell you things without caring if the thing will die or not.
Some questions that can help test if the store is decent:
Can I keep this yellow tang in my 30g? (No, yellow tang needs at least 4' of straight swimming space, many other tangs need more)
Can I keep a bunch of Baggai cardinals/green chromis together? (this is tricky. Both species supposedly school but will actually kill each other off. If they answer 'yes', that's incorrect but alright. If they answer 'no', you have somebody knowledgeable.)
Can I keep this mandarin? (Only in an aged tank that's at least 75g with a fully functional refugium)
Can I cycle the tank with fish? (yes, but it's cruel. Don't do that.)
Can I keep firefish together? (usually they'll kill each other in anything but a large tank. Pairs form, but rarely)
Will this hermit crab kill snails? (yes. Some are less prone to doing so, but yes)
Can I keep this linkia/tile/fromia/sand-sifting starfish? (no, it'll gradually starve, and the last one will eat all the good bugs out of your sand while doing it. All need very large tanks and are difficult in even those)
Can I keep this sand dollar? (no, it'll starve in all but the largest tanks)
Are bristleworms bad? (No, they clean gunk out of the tank. They're good. They will, however, sting. Also, fireworms sting badly and some will eat corals. Bristleworms are harmless if you don't touch them)