After completing the registration and signing in, your Fishing Planet adventure begins. The first taste of the game comes through a short tutorial that'll give you a basic understanding of the fishing process and interactions with the map and inventory. After you complete the tutorial (it's an easy one, so don't worry!), you'll already get level 2 plus a few credits and gold coins, as a starting gift. Spend them wisely, because getting broke is the most common mistake among beginners.
It is highly recommended, after you finished the introductory tutorial, to take a look on in-game tutorials. They can be accessed using dedicated button, on game global screen:
Any fish you catch, from smallest to biggest, except unique ones, is worth credits. All you need to do is to catch and keep the fish until the end of the fishing session or until midnight, when fish will be automatically "sold". That's why the first indispensable thing is a fish keeper - stringer or keepnet. When you're promoted to level 2, the game will "force" you to buy FishJail Stringer Short, for a price of 10 credits. The rest of the credits is enough for your first trips to the only available fishing location at this level - Mudwater River, Missouri. During these first fishing sessions, keep in mind this one thing: make them profitable, which means catching enough fish to cover your travel and fishing expenses (20 credits for travel and 5 credits for each fishing day). If you have a bad fishing day, don't leave the location, just use "time travel" feature (press "T" key) and start another fishing day. This way, you will pay only the fishing expenses (5 credits). You can also use this feature to consolidate your bank account, fishing for several days in a row, without leaving the location.
In the beginning, it's better to focus on panfish: they're easy to catch using entry-level tackle and very common and cheap bait - bread or red worms. If you are careful and skilled enough, you will also get several bronze achievements, like Lucky Bobber, Angling Machine or Panfish Overlord, what means small amount of credits and some experience points. After a few fishing sessions, once you acquire some basic skills and a get a more solid bank account, you may spend around 130 credits to buy your first spinning tackle combo (a spinning rod + a spinning reel) and a few lures (small slop and casting spoons) and try to catch Largemouth Bass and Grass Pickerel. A cheaper alternative is to buy bigger hooks and some specialized bait - cheese or pet food - and try Channel Catfish. In both cases, the rewards for these fish are bigger, so financial problems will be solved in no time.
Meanwhile, the gained experience, from catching fish and from first achievements, will allow you to grow to higher levels and soon you will be able to explore new fishing locations. Make sure to go to the store and buy a license before your first fishing session on every new location. Catching fish without a valid license or keeping unlicensed fish, implies penalties and you will lose some of your credits! Descriptions for available licenses and prices can be found in the Store or on the pages about the fishing locations. It's probably better to get a license for the number of days that you actually plan on using it. Don't buy long-term licenses unless you're going to make the most of them.
When buying new tackle or equipment, think before spending money on things that will be rarely used. Have a clear plan about what and where you want to fish and act accordingly.