If you're going to be focusing on Melee, I'd focus on all 3 of the Physical upgrades, Speed, spells like Physical Shield, Slow Time, Berserk, and/or Multi Strike, as well as a few Magic Power upgrades.
If you want to focus on Ranged fighting, which isn't recommended, start with things like Accuracy, Physical Shield, Slow Time, Multi Arrow, etc.
And if you want to go mage, just go nuts with spells! Feel free to ignore most of the physical upgrades (at least, at first), and focus on putting all of your experience into spells and Magic Power. I'd personally go with Physical Shield, Lightning, Force Push, Slow Time, and Heal Life to start.
There are also some other little things worth noting about upgrades:
• Physical upgrades, like Physique & Toughness, change your physical appearance the most. You'll become a bit larger, and grow more hair on your chest with each level.
• Raising Guile grants you the "Steal" expression at Level 3, and the "LockPick" expression when maxed.
• A handful of spells come with an alignment modifier attached, with a +/- notice in the description, as well as a color indicator (Blue or Red) in the level bar. So if you're an Evil character, for example, putting experience into "Heal Life" will move you closer to a Good alignment; vice versa for spells like Berserk & Drain Life.
• Certain spells can only reach max level if you're of the proper alignment. They'll mention this in the spell descriptions.
• Higher levels of spells can target multiple foes at once, so it's worth sticking with one you like, instead of spreading your experience between a few spells.
And lastly, a quick way to earn spare Will Experience is to get your combat multiplier as high as you can with either melee or spells, and then repeatedly use a low-cost spell like Physical Shield. Each casting takes only a little bit of MP, and only grants you something like 5 Will Experience, but if you have a high combat multiplier, you can get a couple thousand experience out of a single magic bar.