When soloing large groups of bandits early in the game, you need to take your time and use your horse and it's manoeuvrability to your full advantage.
First look at the battlefield and find a good open area which won't slow your horse.
Then lure the bandits into that area and engage them at the extremity of their ranged weapon range, i.e. constantly ride circles around them and force them to use up all of their range weapon ammo trying to hit you (you'll take a few hits, but 99% of their shots will miss due to your speed and their poor skill).
During this stage of ranged combat, you'll want to ride around them in an anti-clockwise circle, keeping them to your left at all times and this will also all you to fire from horseback with your ranged weapon (which can only be fired to your left on horseback).
Your constant circling of them will cause them to bunch up like a bait ball, so when they're bunched together let off so Hail Mary shots with your range weapon (which will probably miss due to your low proficiency and lack of Horse Archery skill, but you should get a few hits and kill a few of them, plus scoring lucky hits and headshots whilst moving on horseback will increase your proficiency by a large amount at low levels).
Once both you and them have run out of range weapon ammo (you'll notice that they'll draw melee weapons and start to charge you), start circling them clockwise and keep them to your right, so you can attack them with your melee weapon. Don't charge into the group of them, but instead keep circling and only aim for individuals who begin to break away from the group, or after you've struck down a few, individuals who run away and try to flee the battlefield (who can be ran down with impunity).
At low levels, despite having poor weapons and proficiency, you still should be able to cause a lot of damage if you strike from horseback and aim to only hit them with the tip of you weapon, as your horse's momentum is added to the power of your swing, and most weapons cause more damage if they hit with their tips and the end of their swing.
Soling large groups of bandits early on is a time consuming process, but you'll gain a few levels per battle and will get plenty of loot which will be superior to your starting gear.
Also, before you set out in the world, head to the first town, go to the arena and participate in the fight a few times to gain a few levels and increase your Strength attribute and Ironflesh skill, which will increase your maximum health.
If you manage to be the last man standing in the Melee Fights, you'll get a bit of money with which to either buy some better weapons (go for the longest weapon you can afford if you intend to solo groups of bandits from horseback, even a low quality broken spear can be deadly when thrust from a charging horse, plus it's range and the height you get from being mounted should mean that your enemies on foot will never be able to reach you with their short weapons).
Another priority should be to go to the Taverns and recruit Hero charters. These are permanent companions who can't die, so it doesn't matter if the get defeated in battle a lot to begin with. Some will join you for free, some will demand a bit of money, but when you're starting out, even the costliest companion is worth grinding the area for enough money to hire them, as they'll always have better starting equipment that you, and maybe a good horse, which you can take of them. For example, Nazeem may cost something like 300-500 to hire and be insufferable, but he rides a 1500 horse, has a robe with a lot better armour than your starting outfit and wields a scimitar which will have greater range and be a better cavalry weapon than your starting sword.
