Expend some time to create your character. Thoroughly read descriptions of classes, races, origins and their biography. You cannot undo your decisions.
Give various roles to different party members. Try to have at least one person that specializes in mechanics and stealth. It is good when each of your companions have a specialization - if you're not using it directly (like mechanic for lock picking), it doesn't mean you're not using it in different way than assumed, like athletics to climb on normally unavailable walls/rocks during scripted interactions.
Develop your character in one direction. At the beginning think about your character development path and pick only those abilities and talents that will suit it.
Don't wait with character developing. Upgrade your party equipment whenever it is possible. Don't wait with using ability and skill points as well.
Don't wait with upgrading your character's equipment. Opening the window and comparing stats of two items doesn't take much time and it can greatly improve your character.
More isn't always better. You don't need to expand your party to six members. If you think that you have created a perfect 4-person team, then don't change it. Gained experience is split between all party members. The less members, the quicker they will level up.
Companions that you met are not the best choice for the party. They are not strong. When playing on high difficulty levels, it is more profitable to create new character from scratches than use those predefined.