Having completed the game in NG and NG+ (Way of the Strong) and also having a couple of friends who I've been helping to get started with the game (one is a Souls/Bloodborne player and one has never played them) I thought I'd list out some of the tips I've given them that have helped them through the game.
I've also received a lot of useful info on this subreddit so this is my way of giving back.
Do the tutorial mission at the start.
Seriously, do it. You need to understand the game mechanics, specifically Ki pulsing, purifying yokai realms etc.
Take it easy, check out the area before you run in, and bait individual enemies so you aren't fighting a mob all at the same time.
You don't need to rush, it's not a race. The beginning of the game is all about learning the mechanics and understanding how enemies work. There's nothing wrong with strafing around enemies, waiting for them to attack and then running in for a single hit and backing off again. Leave complex combos until later, learn the basics first. Also…at the start of the game equip 'stones' to your shortcuts. You can lock onto enemies, chuck a rock at them and they'll then come to you, so you don't end up aggro-ing a whole group. Also, keep an eye on your Ki/stamina bar (the green bar at the top under your health that regenerates automatically). DO NOT RUN OUT OF STAMINA. If you do, you will be stuck on the spot for a few seconds (which is an age in this game) with no attacking or dodging ability. You will most likely die in this state as enemies have an open attack opportunity.
Do lots of exploring.
You'll get more gear, items and amrita (the stuff you use to level up), but also exploration usually leads to shortcuts back to shrines, which means if you die it won't be as much of a slog to get back to your guardian spirit and regain your amrita.
When you're levelling up, put some points into the 'Spirit' stat.
This unlocks the passive abilities that your Guardian Spirit has. You can see the passives easily by looking in your status menu, or by going to the 'Change Guardian Spirit' menu at a shrine. You'll se a list of abilities that you can unlock by having a certain rank in the 'Spirit' attribute. I recommend getting 'Spirit' to at least 10 to start with.
The weird red swords sticking out of the ground everywhere (if you're playing online).
These are Revenants. A revenant is created every time someone who is playing the game online dies. If you look at these revenants you'll be able to see what they had equipped (armour and primary weapon) when they died. It will also tell you how they died. These are useful for a couple of reasons. Firstly, you can fight them (not actually that person, just a computer controlled version of them) for their gear. If they are a higher level than you then it's likely they have higher level gear. Gear level and character level are not linked – you can equip level 50 gear at character level 1 if you wanted to (more about character levels and gear levels below). Secondly, there will be a note telling you how they died. Which will give you an indication of what enemy you have coming up so you can prepare yourself a bit.
Character levels, gear levels and mission levels.
Your character level and your gear level are not linked, You can equip any level gear at any character level. Generally speaking, your gear level is more important than your character level. Better gear = an easier time in the game. And at the start of the game, you generally want to keep swapping out your existing gear for higher level gear as you play. When you progress through missions, you may notice that on the map screen each mission has what looks like a level requirement. This is a guide and shouldn't be taken as an absolute requirement. My advice is to compare your gear level to the level of the mission. If your gear levels are within +/- 10 levels of the mission then you should be ok.
Armour types and armour sets.
There are three types of armour in the game. Light, medium and heavy. There is also something called an agility raring. When you equip new armour you will see three little armour icons with either 1, 2 or 3 of them shaded in. This denotes the armour weight. You will also see that there is a percentage number that will be coloured blue, green, yellow or red. This is your agility level. Blue means you're super agile, red means you can barely move. This has a HUGE impact on combat. If you're agile (in light armour) you can dodge further and attack more frequently with light weapons. If you're in heavy armour, blocking takes up less Ki (stamina, the green bar at the top), but just about every action you do takes up a lot of Ki, and your dodge rolls will be slow and you won't roll as far. You can put points into the 'Stamina' stat when you level up which will increase the amount of equipment load you can handle. This will bring the equip load percentage down.
Pressing the 'options' button in the menu.
This will give you a help popup which will explain the different menu items, stats, abilities and attributes. The game is overwhelming at the start, so use this to get a handle on what you are looking at.
Look at items in your inventory.
A lot of the time they are helpful. Specific mentions: Stones, Salt, Summoners Candles, Himorogi Branches, Talismans
You unlock new weapons, ninja (ninjutsu) and magic (omnyo) in the Skills menu.
Take a look at these and don't neglect the Ninja and Magic skills. Specifically useful ones in the early game are the sneak attack/backstab one from the Ninja skills, and the devigorate and weakness talismans from the magic trees.
Adapt, experiment and trial and error.
This whole game requires trial and error and adaptation. If you are dying a lot in one place then stop, think about what has and hasn't been working and try something new. Look at your skills and items to open up new potential strategies, analyse how you've been playing. This isn't your typical 'run around like a loon and button bash to win' game. It requires patience and the ability to learn from your mistakes.
A note about the early game bosses.
The first two bosses are a bit of a trial by fire in this game. You may find yourself wanting to throw your controller through the window. Take heart from the fact that they are possible to beat once you are familiar with their game mechanics. And the feeling of accomplishment when you do beat them is pretty good. I'm not going to talk about boss strategies here, there are plenty of posts on that with great advice (just search this subreddit for the name of the boss and read the comments in the posts).
Those are the main bits of advice, take on board what is useful to you.
There are also a few things that I constantly shout at my friends when I watch them play. Some of them are very much common sense, but I still have to remind them!
Fully exit out of the menu before you start running around. You can move around with the menu open, but you won't be able to attack. Enemies will mash you if you do this.
Block attacks a lot, but don't hold the block button all the time. Your Ki regenerates very slowly when you have block held.
Keep an eye on your health! Back off and use an elixir at an opportune moment. Nothing is more frustrating than getting an enemy down to a sliver of health and then getting killed by the tiniest little crappy attack because you only had a little bit of health left yourself.
Don't be greedy with your attacks. 9 times out 10 this will bite you in the arse. That extra hit instead of backing off or dodging will get punished.
Hope this was useful to at least someone. Hit me up if you have any questions or feedback.