With BD's stat system, there is a great deal of flexibility in creating viable character builds. Some people have played using two melee warriors the entire game, but one warrior and one archer or mage is probably more common. I don't recall anyone trying two mages or a mage and archer.
The math appendix in the Strategy Guide (which Flixerflax gave a link to) describes (among other things) the stat effects, and gives some general advice on stat point distribution.
For the majority of the first act there are lots of skeletons around, which are more vulnerable to crushing damage. Until I reached the citadel (late in the first act) I had my hero use crushing weapons (a club or 2 handed walking stick) most of the time and a bow for tougher fights, then I switched to archery full time. My DK continued to use slashing damage (1 or 2 handed swords).
The DK worked fine as a melee character, but I would do it differently playing again. Since his armour class goes up with his character level, he will not need as much strength for armour requirements, and can concentrate more on intelligence (for a mage) or agility (for a bow archer). Having the DK use a strength based weapon negates this. Since the human will need to meet strength requirements for better armour anyway, this character is slightly better suited to be a melee warrior. This is a relatively minor difference, though, so a human mage / DK warrior combination is just as feasible.
I would say just be a little conservative with your skill points, and you will have a fair bit of flexibility in developing your characters. You can also unlearn skills, at a cost in gold proportional to the square of your character level.
Just starting off you will probably need to go melee/melee most of the time. I have not played a mage, though, so am not sure how effective they are at the start of the game just using magic. Much more powerful and mana efficient spells start to become available starting in act 2 (particularly Shaman magic, as Flixerflax mentioned). Until then, a bow could be a handy backup weapon.
Most weapon skills you can get a good effect with just a skill point or two. For my archer I maxed Reload Time (which gives a direct Speed boost), but for the rest I just put a point in damage and a couple in accuracy.
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