At high-levels, the mage class becomes the most powerful build in the game. That end-game power comes at the cost of being very weak in the early game and difficult to balance. Players often find their mages have a huge advantage in an area, or really struggling.
The skills of a mage, mostly spell bases, also take a bit to learn how to use in combat to the best effect. For example, learning how to bounce Ice Storm off walls can result in repeated high-damage hits on a monster. Or, you just can miss completly and wondering where your mana went.
There's no reason to have any investment in STR as a mage in the early, or mid-game. (And very, very little reason in the end-game).
As a mage, you should ONLY join the mage guild. Unless you are tring to do a purposefully handicapped run through the game, you'll find no benefit to the other guilds.
IF--and that's a big if-- you are still learning the game, I -might- suggest you start as a Cleric, with the following mile-stones. Get Heal, Crusader's, at least one point in Intervention, 1 point in Might, (Might is very powerful in early game, with little mana cost), join the Warrior's Guild for Duel. Later invest in Heavy Hand. Flush out other skills as you desire (including opting for more intervention, etc).
Invest in STR till Level 4 (for heavy hand), invest in PERS and INT for some mana). Maybe get AWA 2, END 2 for durability.
Between heal and intervention, you'll have a lot of health to use. With Duel and Heavy Hand you'll damage to balance out the Cleric's limited weapon choice. You can swap between shields and 2-H weapons as needed (I prefer shield with 1 hand).
That build might be the most forgiving in terms of tweaking exact builds. It will give you a lot of options in quest choices (PERS and INT will open up dialogue choices). And will give some experience in figuring out game mechanics.