How do you get 99 str in runescape

By training that skill as much as you possibly can. It will take ALOT of work, time, and patience, so be prepared to put in hours. Also, don't set the goal for level 99 right off. Instead, set smaller goals that are attainable but difficult, so that you can feel good about yourself for reaching smaller goals on your way to getting there. (For example, go for lvl 50, then 55, then 60). Train at Lesser or Greater Demons for the higher levels, as they are great experience.