Hmm... since people have already said go big from the start, research EVERYTHING including the requirements of your fish as an adult, and CYCLE...
Mine would have to be, be very careful around LFS. Most do not give good advice. Most do not have your best interests or your fish's welfare involved. Asking the shop assistant does not qualify as research. (Somebody tried to tell me last week that Pim pictus was a tandanus - a native eel-tail.)
BE CAREFUL WITH LFS. Zitty-faced teenage girls with panda eyes and the sleeves on their uniforms rolled up - NOT a good source of advice about the hobby. Shopkeepers run off their feet trying to keep an eye on the thuggy bloke in the hoodie, and trying to deflect the nagging from that woman who just doesn't want to hear that you can't keep an oscar in a 20 gal, while trying to net fish for a young mother who is unsuccessfully trying to shush a toddler screaming Nemo! Nemo! and thumping on the tanks - NOT a good source of advice. The next door neighbour who has managed to keep two goldfish in a gallon bowl for the last year and they're still alive - NOT a good source of advice.
BE CAREFUL WITH LFS. Chain stores in particular are trouble, especially where fish health is involved. It's the people who dont' know what they are doing who will buy a fish with TB or parasites or columnaris without realising that it's sick. Research diseases before you buy and make sure you choose healthy fish, and QUARANTINE. Do NOT take the word of the LFS that they are healthy. A health guarantee covers ONE FISH. If that one fish wipes out your tank - it's your bad luck.
BE CAREFUL WITH LFS. Even the guy with the shmicko tanks chock full of beautiful fish may not give you the right advice. A common trick is the 'chemical for everything' and some of them believe what they're saying. Ammonia problems? Don't bother fixing the filtration fault or working out why your tank isn't cycled. Just add this. Fish are sick? Tip in this. Algae? Add some of this. Plants look sick? Here's some magic potion. YOU DON'T NEED THEM. Many of the most successful fishkeepers use nothing more than water ager and some don't even use that (leaving the water to stand instead). You also do not need a CO2 unit, a push-button canister filter on a 20 gal tank, a cute little fish that will be a tank buster, etc. Don't be talked into it.
BE CAREFUL WITH LFS.