Fish you just walk up to the right spot and press A or X (Xbox/PS4 controller). Small animals you equip your capture net and aim it with L2 and shoot with R2, by default it should be on your item bar. Neither of these things is important or necessary, so I don't know what you're doing where it's asking you to do it (fishing can get you really good whetfish but the easiest way to get them is to NOT fish and just shoot a net at them...you literally never need to fish ever). tl;dr DON'T WORRY ABOUT FISHING OR SMALL ANIMALS, IT DOES NOT MATTER
Capturing large animals matters. Instead of killing monsters, you can put a trap down and throw tranquilizer bombs at them instead. They have to be low HP, nearly dead, so usually if you see them start to limp and run away to their lair to sleep, they're capturable. Capturing versus slaying can get you different rewards (sometimes), but capturing ends the fight faster so it's often a good option. You can only carry ONE of each of the TWO trap types each mission, so traps are kind of limited without making more or going back to base.
Crafting! It's important. You can set things up to autocraft (triangle, I think?) and the game just straight up gives you all the crafting receipes...this is FUCKING NUTS cuz in old games you just had to KNOW what items combined into what, you couldn't autocraft (you can set shortcuts to craft items MID FIGHT!!!!), and it used to take more items (Potions were herbs + blue mushrooms, not JUST herbs being 1:1 with potions, for example).
Anyway, old man complaining aside, crafting is important. Early on you mostly just need to do it to make sure you have enough potions, but eventually you're gonna want to run with more stuff. Don't sweat it too much for now, because there's a lot to take in re: consumables and inventory management. Just play around with crafting stuff for now.
You can craft from your item box which is basically mandatory.
Anyway anyway, fuck all that, if you're gonna rock Dual Blades, watch this video:
He has one for all of the weapons and the additions they got in Iceborne as well, so I recommend that too. I actually don't like DB at all, so I can't really give you an personal tips other than I think they suck. I mean, they don't suck they're good, all the weapons are viable and good, I just think they aren't fun.
The game starts you off with one of each weapon type, so I highly recommend experimenting and finding something that fits you rather than just using DB because someone else said they're good for beginners. For instance, my favorite weapons were Lance, Gunlance, and Hunting Horn and those are consistently 3 of the least played weapons on each platform (HH is consistently last and has been in pretty much every game in terms of popularity, which is nuts to me because it's INSANELY STRONG). I started playing Light Bowgun a few months ago and liked it, and back in vanilla MHW I mained Bow for a while and was just terrible with it but still liked it. In MH4, I started with Charge Blade before I found my jam with Great Sword. In Iceborne I one day arbitrarily made a Long Sword set, and it's one of my highest DPS options and I love playing it. Experimenting with new weapons is fun, but I do think it's helpful early on to pick one and stick with it for a while to gain experience with it's moveset early on.
Good consensus "Beginner" weapons are the sword and shield, dual blade, hammer, long sword, and greatsword...but I started learning MonHun with the Hunting Horn, so fuck it anything is possible. The guns are tricky to use because they require managing ammo and using gun mods, but the actual weapon mechanics are the simplest of everything; you're basically just playing a shooter with them with some weird mechanics tacked on. Any weapon is viable and learnable, it's really just a question of your playstyle and preference. I think this video helps explain what the different weapons REALLY do.
It's in the context of MH Double Cross (the Switch one that's a sequel of sorts to the 2nd 3DS one) but the general tips still apply; that's just why the video looks different. I do agree that GS and Hammer are probably the "Best" starting weapons, but also Lance fucking rules don't listen to the haters. It's considered a "Defensive" weapon, but instead you just do your counter move and armor through everything and never stop attacking (GL can't do that, but it can BLOW SHIT UP instead so it evens out).
If you got Iceborne, there should be a weapon set and armor set called the Defender or something like that; use it. It's designed solely to carry you through the main game without taking very long to make, so definitely take advantage of that.