This is my first ever game of NWN despite having the game for a good 2 years now lol. To go easy i decided id take a straight fighter DW katanas. hes level 10 now and i feel i want to add a little bit of power to him by letting him use arcane magic, the problem is im not sure the best way to go about expanding the build. should i just take 1 lvl in wizard or sorcerer to be able to use spell scolls for a little added boost or should i level up the 2 classes evenly.

cheers.

Taking a second class now will affect your xp, until, or unless, you catch the second class up to within one of the first class. This isn't an issue with human chars, no preferred class, unless you are your races preferred class. Not sure from your post what race you are running though.
If it were me, running in a sp campaign, I'd go ahead and take either sorc, or bard, and up my lore skill to at least 8, catch the lvls up, and take 10 lvls of RDD. If you go bard, be sure to raise your perform skill, the bard song gets better if you have more points in it.

sorry fogot. hes human

What are your WIS, INT and CHA attributes?

INT - 16
CHA - 8
WIS - 8
(powergamer)

Alright. First off, if you're only interested in using spells from scrolls or wands, then you'd do better to take rogue levels and invest in Use Magic Device. In your case you'll want to invest about 16 points into the skill, taking between 1 and 3 levels of the class. An added bonus is that UMD does not work for wands and scrolls alone, but also lets you use items restricted to other classes, races and alignments regardless of type and purpose.

The advantage of that approach is that this will hamper your combat skills far less, and it'll even add some bonusses in the form of sneak attack (+1d6 or 2d6 damage if the person you attack is not currently fighting you), evasion (allows you to avoid damage from most traps and elemental spells on a succesful reflex save) and uncanny dodge (lets you keep any dexterity bonusses to AC always). You also get some other rogue advantages but they're trivial in this case.
The downside is that most spells cast this way are nowhere near as powerful as the spells you'd cast as a wizard. They do less damage, are more easily stopped by spell resistance and saving throws, and don't last as long.

If you are going to take caster levels, then with your stat line it will have to be in the wizard class. Sorcerers and bards, as well as divine casters like rangers, druids, paladins and clerics, don't have much potential with your point distribution.
Whether or not to make that change really depends on what you want out of your magic. Wizards don't really come into power until later on.
For respectable buffs you should go for level 5 spells (9 class levels). For even better defenses, you need spell level 7 or 8, or even epic spells. Spell level 6 is when you'll start getting the more lethal magics, this carries on right up until 9th.

Focussing on the fighter character, you'll probably want to stop sooner. Spell level 4 is a good choice then (7 class levels).
The other option is to take just the one level afterall, and to use spells like Identify, Truestrike. Your familiar can be used for distractions and trap disarmament.

Whatever the choice, with 10 figher levels already you don't have to worry about losing attacks per round, you'll have the maximum number of those.
I personally find that the multiclassing of fighter/mages doesn't work too well in most situations so I'd opt for the rogue levels; in the OC there are plenty of immunities and advantages to be had through items, as well as a decent supply of scrolls, wands, potions and other class-restricted items.

Fighter/mages don`t work well, but oh, cleric/weapon master works wonders....

Sorry this wasn`t really helpful.

What may hamper your magics is your armour. The chance for Arcane failure is fairly high when wearing heavy armour. So if your fighter is that sort of person str based as it seems, (DW katanas) you may have even more trouble casting. I wonder if the Rogue`s Use Device has an arcance failure chance?

does casting from scrolls also have arcane failure chance? and what about wands?

No. Though depending on the module, you may need to make an UMD check to use the scrolls (not the case in the official campaigns).
Wands and scrolls, like potions, are unaffected by the armor you wear.

Spell failure wont be that much of a problem since im wearing light armour. i have quite a considerable dex bonus and heay armour lowers my AC