A bit late, but I'll add a few tips.
When creating a character (this is probably too late for you now, but anyhoo) Strenght is very important, even for a mage, as it determines your carrying capacity and, trust me, you'll need every ounce of it. If you want to wear heavier armor, you will need to have STR over 20 (for plate armor, I'd recommend STR 24ish) to be able to wear it and haul some loot with you. Sure, you can drop items and create ad hoc warehouses for yourself wherever you want, but eventually it will get tiresome to run between levels to get that one item you need, that you don't have on you right now.
If you play a mage, you will need good Intelligence to up your mana pool (which powers spells) and base Casting skill. However, you can improve your mana in-game, so INT is not as crucial early on as good STR, in my opinion. YMMV, depending on your character build. Be that as it may, a good INT is required for any character wanting to cast lots of spells and/or higher circle spells as the higher circle ones cost quite a lot of mana (3 x Circle, IIRC).
Contrary to most games, in UU I've had DEX be my "dump stat" as I honestly didn't find it that useful.
Keep in mind, that you cannot improve attributes during the game, so be sure you're happy with them before venturing further into the game.
As to skills, I found that (sadly) most of them are pretty useless. As is obvious,
Attack and
Defence are of course important, as is a
weapon skill (pick one and stick with it). If you want to sling spells,
Casting and
Mana are important, but you can get by surprisingly well with high INT and only some Casting on the early levels. Besides, you cannot cast spells higher than half your level (or something, can't remember now) anyhow and you're limited by the rune stones you can find, so focussing on Casting before the 3rd dungeon level is a bit of an overkill, IMO. (Again, YMMV on your character build. I've never played a pure mage in UU1.)
Lore skill is useful for identifying magical items and potions, however, some items require really high skill to identify (22+), so it might be good not to go overboard with this and rely on identifying spells for those (Caveat: Identify is a 5th circle spell, so that's a problem for fighters etc.; there are few identifying scrolls around, though). Of course you can just cheat by abusing saving and use whatever you find unidentified, find out its effects and reload. This won't work for magic weapons and armor, though.
Search is also good to have a bit, to help you find secret doors. Last but not least, a few levels of
Acrobat and
Swimming will help you out some, but are not really required. I never did find any good use for the other skills. Missile weapons might be OK, but I found them cumbersome to use and ammo scarce (except sling stones). Repair might be OK, but it needs an anvil to perform and there aren't many of those around, plus you'll find someone who repairs items for you for a modest fee pretty early on (and gives you something to spend your gold on, don't forget to tip him
).
This I've talked about before, so sorry for the repeat - remember to NEVER EVER nest containers, i.e. put a container inside a container. THis has been reported as one of the likely reasons for the infamous inventory bug (even with the patched version). I've have had the bug stike me, even when I was not nesting containers, so I'm not sure how accurate that cause is, but it pays to play it safe nevertheless. Also, save before every dungeon level change. SOme people have had problems occur at/right after level transition, so saving right before one is a sound precaution.
This is common sense, but maybe it's worth to note - do not attack anyone not attacking you. Most people are important one way or another, so killing them out of hand is very ill-adviced (and can break the game). Try to talk to everyone (well, not everyone, not the rats, hostile goblins, etc.).
Gotta go, at work.
P.S. A good 1st circle spell for every character is BIS which gives you extra defense/armor and the best 2nd circle spell is IBM which heals you. I found them both very useful early on. OJ (1st circle) is alright too, your basic magic missile that, unfortunately a bit a low in damage.