As a Witcher, you're one of the greatest swordsmen and most capable warriors in Temeria.
You're also a Level 1 character in a role-playing game. You don't get to have everything your own way, at least not at the start. You may have access to all five of Geralt's major Signs, as well as traps and helpful potions at your disposal, but you can't take many hits and have to be careful about the encounters you get into.
For the first few levels at least, get used to relying on your two swords - the steel sword for humans, elves and dwarves, and the silver sword for everything else. Using the wrong one means you won't deal effective damage. Look at the combat log in the bottom right of the screen for warnings and switch blades accordingly. Never go out into the wilderness without a silver sword on standby, even if it means stealing a whole village worth of coins to buy one. (Nobody will complain.)
Your most useful Signs at this point in the game are Aard - the knockback and stun attack - and Quen - a personal shield. Of the others, the fireball Igni doesn't do much damage, the mind-control spell Axii takes a long time to kick into effect, and while the Yrdren spell can be useful (especially against one early boss), you're usually better putting down an actual trap that causes damage if you have the time to set one. The exception is when you're facing multiple enemies as you enter an area, who automatically attack and leave you no time to place one. In these situations, your foes usually cover the ground between you too fast for you to place the trap and get out of the blast radius.
Alchemy is incredibly important to a Witcher's survival. Early in the game, you should always have a few Swallow potions prepared, which you can take before going into areas where you know there's going to be a fight. The ingredients for this are everywhere and it lasts 10 minutes, so there's no reason not to drink them down until little bubbles of the stuff comes frothing out of your nose. The only other potion you need at this point is Cat, for seeing in the dark and spotting the occasional clue, although you won't hurt yourself by experimenting with the others. You can have up to three potions active at once.
Even at this point, one-on-one fights are rarely an issue. Your main threat is from groups, who deal much more damage when they hit you in the back, and will inevitably land more hits. You can block, but that drains the stamina you could be using for magical spells. Your best bet early on is to do hit and run attacks, with as much crowd control as possible. Roll constantly and make heavy use of Quen against large groups of weak monsters (such as Nekkers), and Aard against armoured enemies like nights and Scoia'tael soldiers. Wherever possible, finish off enemies quickly instead of trying to chip away at all of their health. It doesn't take many successful blows to kill soldiers, but the same goes for you. A couple of critical hits will obliterate you in a matter of seconds, Witcher training or not.
In the opening chapter, your first real challenge is a courtyard full of knights in the fourth section of the flashback. This is a massive difficulty jump, and one that you might not be ready for. It's worth persevering, but if you find yourself being frustrated, consider temporarily dropping the difficulty down to Easy and back again after you win. Not only does the first chapter gives you plenty of time to learn your skills, much of the difficulty isn't likely to be down to your bad play so much as not having specific talents at your disposal. By Level 10, most fights are much smoother. Even if you plan to follow the Magic or Alchemy talent trees, it's worth investing a few of these early points into swordsmanship - particularly the talent that removes the extra damage you take from backstabs.