Some advice:
Always check the bestiary. Kill an enemy once and it'll update with that type's information, including its weaknesses (signs, bombs, oils).
Learn how to best use the two tier evade system: roll uses stamina, and is best used to escape/create space in an emergency. Dodge when an enemy attacks to create an opening for attack. Dodge uses no stamina, so try to stick to this as often as you can.
Make use of potions as often as you can. Two of these best for early game are:
Swallow - health regen
Tawny Owl - Stamina regen
Make oils as soon as you can, maybe even spend some time hunting down oil recopies. Remember you can dismantle items for specific ingredients if you're having trouble finding them.
Crossbow knocks things out of the sky/one shots underwater enemies.
Don't spam attack, enemies will punish you. Create space, strike 2-3 times, dodge away/counter.
If you're really having trouble then find a group of tricky enemies (wraiths or drowners are good for this) and practise evading until you can survive indefinitely while surrounded. one you have this down, start introducing strikes and other attacks.
It can take some time getting used to it, be patient.