Instructions
Begin with a character class that offers a strong Strength score. Melee combatants like Dark Knights and even Magic Gladiators are good starting platforms for Strength stats.
Increase in level. You earn experience points through fighting monsters and completing quests, and when you gain enough experience points you go up a level. When you rise in level you will have the opportunity to change your stats, enhancing your character. Put points into your Strength score if you consider it important.
Use items that enhance your Strength stat. Magic items of all stripes can add to your stats, and when you find a magic item that increases your Strength, simply equip it if you're able to use it.