Here are some helpful tips for your first little while of playing. Warning, incoming wall of text approaching:
When you first start out, shake all of your fruit trees and look out for the perfect fruit. You can also stack fruit on top of each other to make baskets so you can carry and sell more. The perfect fruit will be your town fruit but will have a slightly different appearance. Keep it, plant it in a good spot, and you can grow a perfect fruit tree (note, you can only grow perfect versions of your town's native fruit).
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This game introduces ore, and there is a fake rock in your town every day that when you hit it, it reveals an ore. Also, hit every rock with your shovel, as you may find the magic rock that drops bells when you hit it. If you have a silver shovel (you can buy it from the museum shop after its unlocked, which happens after donating 50 items to your museum and having at least one of each type of item donated like art, fossils, bugs, or fish), sometimes the rock will drop ores instead.
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Always always always ALWAYS sell to retail, you get more money there as apposed to selling at the nooklings store. Make sure that before you sell things like bugs, fish, fossils or art you have donated them to the museum first, that way you don't have to worry about trying to find them all again.
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When you start, try to buy all the tools right away, especially the fishing rod and bug catching net, so you can start to make money easily. —————
After three days (I believe) of playing, you will unlock tortimer'a island. You can travel there to get exclusive items, furniture, fruits and flowers. It is also a money making gold mine.
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To make the most money on the island, talk to the gyroid there and rent a shovel, an axe, a fishing rod and a bug net. Use your axe to cut every tree except for the ones on the sides of the island and dig out the stumps. You can also plant more of the beach trees on the side of you want to using the shovel, they just won't be ready for three days. next, remove all flowers and bushes using the shovel. Once your landscaping is done, return the shovel and axe to the gyroid. To make as much money as possible, go to your new refurbished island after 7 pm, and get a net and rod. Walk carefully around the island and catch any bugs you see on the trees and catch any fish shadows with fins sticking out of the water (turn your volume up or use headphones when fishing, the actual bite noise is a more distinct noise from the nibble). Put all your catches into the box near where you came onto the island and when it's full, leave. Sell everything to retail, bank the money, and repeat.
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Also, look out for tents or people near your town tree, mayor's building, or just wandering around. The tents will always be near your tree, the purple one is a fortune teller's tent, and for 500 bells you can find your lucky item that will increase your players luck (there is a guide I believe that lists what type of item increase what luck, but I don't know off the top of my head. Look up animal crossing new leaf luck guide and you should find something). A green tent is where you buy art, although sometimes it is fake, using guides or having art knowledge helps here. Isabelle will be at the tree on certain holidays (groundhog day, nature day, etc etc) and will be near the mayor's building if that holiday coincides with another event (like fishing tourneys and bug tourneys). I don't know the the schedules for the tourneys, as I have not participated in them often enough, so you may want to look those up as well. Also, during Easter someone named zipper t bunny will visit and want you to find eggs for him, then there is pave who does the same thing with feathers during festivale (which I think is centered around carnivale in real life), and there are the snow people during the winter. The snow man plays bingo with you, the snow mam wants you to collect snowflakes, the snow boy gives you furniture and the snow tyke gives you stuff based on the presence and appearance of his fellow snow family members. Then on Halloween and Christmas, you have Jack and Jingle who will be wandering your town up until those holidays and want you to do things for them. Every event gives furniture (except for the fortune teller or the art dealer, unless you count those works of art as furniture)
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Also, make sure you buy everything you can from the nooklings store, as you will be able to increase your shops size if you spend enough.
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Also, to tell if there is a new message on your bulletin board, look for a little yellow bird (day) or a grey owl (night). Those birds are like a notification telling you that you have a new message there.
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Sorry for the long post, but I hope this helps you out! These are just some tips, if you want more, I am sure others will gladly help you out or I could even spout off some more. Also, sorry for any grammatical errors, I am not super great with punctuation.