The game does a really bad job of explaining anything, which is absolutely part of the charm but frustrating to begin with. I ended up starting again after beating the second boss as felt like I’d fucked up my build already, but in hindsight wasn’t really necessary. You definitely don’t need to do it for the starting items, only one or two are really worth it and even then they absolutely aren’t essential.
To add to what @vamos said, there are two forms of humanity - hard and soft. Hard humanity sits in your inventory, and then when you consume it, it adds to your soft humanity which is the number in the top left of your screen. You lose soft humanity when you die (and get it back when you reach your bloodstain along with your lost souls), and then have to consume one to turn human again. So basically, keeping it in your inventory as hard humanity keeps it safe and consuming it puts it at risk.
Having more soft humanity has some small benefits - curse resistance, increased item pick-up - but probably not worth the risk of dying and losing it all. Really the most important thing is being able to turn human which takes 1 humanity. It’s a big risk/reward thing, as being human will allow you to summon help which can be invaluable for the tougher bosses, but also leaves you open to invaders. There are certain areas that are worse for invasions than others so just a case of getting a feel for it I reckon and whether you feel it’s worth the risk for that particular boss or area.
There are certain enemies you can farm to get humanity, but best way to get it quickly is to stick your summon sign down outside a boss and get summoned to other players to help them out. If you both beat the boss you get some humanity as a reward. Also helps you get a feel for the boss before you try it yourself with little risk, as you don’t lose anything if you die as a summon.
Humanity aside - the systems I didn’t understand at first were the stats and weapon/armour upgrades. Gonna spoiler each section in case you want to work it out yourself -
Re: stats, bottom line is if you screw up your build by investing in the wrong stats, you can always farm those souls back so it’s not the end of the world. But look at the weapons you ideally want to use and what stats they scale best from (these are the letters that each equipment has, stats with a high rating like A or S being the best for that weapon) or the minimum stat requirement for that weapon or spell.
For weapons, it’s really just down to what category (e.g. spear, axe, great axe, katana) you like using and the playstyle you enjoy, and then finding the weapon with the best stats within that category. Experimentation is encouraged early on before you get into upgrading weapons (more below) but it’s never too late to switch things up.
For armour, you need to think more about weight - the heavier you are the slower you move and slower you roll, but obviously the more defense you have. Your speed is based on what % of your overall weight allowance you go for, so less than 25% is a certain speed, less than 50% another, etc (think those are the right figures but might be remembering wrong). So it’s a toss up between being fast but more vulnerable or slow and a tank, just down to preference really.
Re: upgrading, you’re still a little way off from this so don’t worry yet, but upgrading your weapons is important and often more valuable than levelling up. Just something to bear in mind. (Also don’t upgrade your weapon to Raw, it’s a con!). You can upgrade armour too but I didn’t do this til the endgame when I had upgraded everything else and souls to splurge
Only other advice I’d give is don’t worry too much about dying - can be demoralising to lose loads of souls, especially after a boss if you’ve not reached a bonfire yet, but you can always get them back eventually!
And actually - if you’re ever really stuck on a boss, give one of us a shout and someone will definitely be up for jumping in and helping you out as a summon!
(Been a while since I played so big caveat on all the above that I might not be remembering correctly…)
