You can't, really, the idea is that when you roll the dice for a character, you try to get the best scores in the stats that are important for that class. For example, for a fighter you would want a high strength and also good constitution and dexterity, whereas intelligence and wisdom would be much less important.
However, there is a way around your problem, as there are some strength enhancing items in the game:
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The girdles of Frost, Hill and Stone Giant Strength (found at a storekeep in Trademeet after the completion of the quest, in the Adventurers Mart and in the Underdark) There is also a Mace +2 called the Maulers Arm (I think it's in the Copper Coronet) which raises your strength to 18. I suggest that you quickly find on of these items as strength is very important for a warrior. The weigth carrying problem could be solved with a bag of holding: you can put tons of heavy armour in it and it will still weigh a fixed amount of lbs
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Melusine
Your voice is ambrosia