I was looking for a way to increase strength but their isn't a way to do it manually. though I did notice that when I level up it increased by 5. Does it matter if I choose health, magic or stamina? Or will my strength just level up with everything else?

Well, since there's no attributes you can't really gain strength. Since you stated it went up by 5 points I assume you are specifically referring to your carry-weight. Stamina increases your carry weight.

If you are looking to gain DMG on weapons (also previously governed by STR) then you want to increase the perks in the 1st rank of each weapon style.

Thanks. So it was just by chance that I noticed it after I upped my Stamina. Appreciate the help.