Well, there are a few factors to it.
Firstly, you need to check your equipment weight - that is the combined weight of all your equipment (Armor, sword, possibly shield) - while these provide protection bonuses, equipping strong armor also makes you heavy. The heavier your equipment is, the more “clunky” your movement becomes (it takes longer to change walk directions) and your stamina regeneration speed also decreases.
Secondly, the main combat controls are just lmb and rmb
Left mouse button works as normal attack, which is a series of up to 3-4 attack animations. After that there is a brief pause before you can use this attack option again. The attack consumes a bit of stamina, but the amount varies for different weapons.
Right mouse button works as both parry and block - your character is parrying at first for a brief beriod, and after that it changes into a block stance. Just entering the parry stance immedieatly consumes some stamina, but that’s a fixed amount and is deducted only at the start of the parry sstance. After that, the whole time you’re in the blocking stance, your stamina slowly runs out to prevent turtling.
If you get parried, you get stunned and wont be able to move or do any combat action for a short while. Now this applies only if you get parried while attacking, you can also get “parried” by a special attack (more about that down below) which also disables your ability to attack or block, but you will still be able to move.
There is also a thing called a weapon clash, that happens when both players attack each other with the primary attack at the same time. In this case, neither of the players “clashing” will recieve any damage.
Third off, there are special attacks, these are unique for different weapon types and basically hold the role of classic combat skills like in any rpg. They can vary in strenght, speed, stamina cost, etc. but typically have a very high stamina cost and have a several second cooldown. Some special attacks can also break through blocks, cancel parries or even stun enemies just from landing a hit.