When you have gained some experience in that game and you know what kind of teams do different kind of bandits and deserters have then you can try to play solo in the start.
First, you should only attack against looters as Adorno said. You can also try to do some tasks and gain more cash. Your first objective should be to improve your agility to 12 to gain riding level 4.(for heavy warhorse) Also, athletics 4 will be nice but you won´t need it yet. Improve your archery skill, the idea is to kill enemies at the long range and avoid melee fights.
Then your next objective will be to improve your strength to 12 to gain access to warbow, ability to carry heavy armor, having iron hide(?) level 4 and power strike level 4. So you will hit harder and you have more hit points. Improve your armor as well to have mail armor, a metal helmet, iron greaves and some kind of gauntlets. Also, buy a sword for melee fight. I prefer bastard swords since they are pretty deadly and you use them with shields in necessary.
When you have strength at 12 and you have a decently armoured horse, a decent armor, a warbow and a good archer proficiency then you can attack against small sea raider parties. They often have shields so aim at their feet and make sure that you have some extra arrows. Be sure that you keep distance to them. Their archers are pretty inaccurate in long distance but they can deliver quite much damage in point blank range. They will also run out of arrows sooner or later because they shoot quite liberally. Kill they infantry first and then focus on archers. They will give a lot of experience, renown and spoils if you play solo.
Also, use the tactical map at the start and see where your inventory box is in the battlefield. In that way, you can quickly fetch some new arrows. Also, avoid enemy groups with cavalry at all cost.
When you gain experience then you should increase your strength from 12 to 24. Why? Power draw, you need archer level 4 to use warbow and 4 extra level make the arrows to inflict more damage and the arrow trajectory is lower and more accurate because you have more skill to use the bow. That is why power draw is very sweet. You can use any ordinary bows when mounted but you cannot use crossbows.
Also, power strike and iron hide at level 8 are very sweet. You should buy heavy armor, something like guardian helmets, plate mails, iron gauntlets and iron greaves. If they are thick or reinforced the better. You should improve your athletics to 4 as well. Some extra arrows will be useful as well. When you are heavily armoured then you can attack larger sea raider parties. I have been able to kill more than 40 sea raiders in one battle by using my armoured horse archer tactics.
You should also have a faster horse than a warhorse available. A courser or a hunter, a shield and a lance are much better combination to deal with mounted horse archers than a warhorse.