Why don't we start with the lore.
In the late twenty first century, humanity stretched out to the stars, blah blah blah. you've probably already heard the intro, so I'm going to skip all the info that it gives you, albeit it is very little anyway. The mission that the Aurora was sent on was to terraform a planet for human colonization. However, when it was struck by an unidentified energy beam in its decsent, it crashed and you are the only survivor. The mission was funded by the Alterra Corperation, a mining a terraforming company, and this company also appears in certain maps for the game Natural Selection 2. In fact, some of the tools in Subnautica have 'Alterra' written on them. Because the mission was a peaceful one, there were no weapons on the ship, and thus you can only use your knife to kill things in the game. What a shame.

This section will help you a lot if you have absolutely no clue what you are doing. First thing you must understand about Subnautica: Resource gathering is 90% of the game. The other ten percent is hightailing it when a Reaper Leviathan comes after you, and fighting off stalkers. Almost every single tool you can make does nothing but make resource gathering easier. Luckily, each type of resource is gathered in a different way, so it doesn't get boring, but if you like combat in survival games, go play ARK.
Anywho, the resources that are the most useful to have on hand are quartz, titanium, copper, and fish. Quartz, because you can make glass and silicone with it, titanium because literally EVERYTHING uses it, copper because you will need a ton of batteries, and fish because eating has been scientifically proven to lengthen your lifespan. I would recomend keeping somewhere around 20-30 of each on hand in your base, just in case. Believe me, you do NOT want to go looking for quartz in stalker infested water during the night because you forgot to get some to make flippers. It just doesn't end well. Once you have those four essentials, you can go and get some gold, magnetite, silver, and lithium. have about 10-15 of each on hand in your base at all times. If you have these items with you at home, it will cut down the time it takes to make something like a sub by 10 minutes, minimum.

Whats that, oh, you've outgrown the escape pod, and you want to move? Well, its your lucky day, because you can do just that. With just 20 titanium and 6 glass, you can build a starter house perfect until you get a seamoth. All you need to build it is a builder. Builders are relatively cheap, and are an essential, so I recomend getting one as soon as you can. Just go get some table coral, which you can get by cutting it with a knife, some silver, and some copper and titanium. You'll be all set. My recomendation is to make a small base to start with, and once you have a large supply of lithium, (or can get it easily,) make it larger. this is because when you have a large base, you need reenforcments to keep it from collapsing, which require lithium to make.

Once you have a base, lots of excess materials, and have raided the Aurora, you should treat yourself to some special end game stuffedy stuffs, namely lots of submarines. With different seamoths for different uses, you can streamline tasks such as killing stalkers and finding lithium. If you have one amazing resource collection sub, and one amazing combat sub, you'll be better off than if you just had one sub that was, 'meh' at both jobs. Plus, if you have a cyclops, you can make yourself a mobile base and go around doing day to week long resource collection excursions. But, seriously, If you feel like you've done everything possible in Subnautica, just wait a month or two and start a new game. Trust me, it will be a very, VERY different, and probably more fun, expirience than just continuing your game untill you get completely and utterly board of subnautica.