Incoming wall of text! If you want, you can skip down to the bottom for an example starting build.
Can someone suggest me a good level 1 mage build, please ?
Well, get summon monsters I, phase door, knock or wizard lock, and a ranged attack spell of some sort (Magic missile, minor drain, or chromatic orb). Then probably floating disc. And... that's more than you really
need, actually, but covers most your bases. You'll likely have some spell slots left over -- look around, check failure rates (look for penalties -- that means it's an opposed school and you
probably don't want to learn it), and see if there's anything that interests you, grab it, and experiment. You'll probably want to go ahead and grab guildmage and mystic preparation as your initial feats -- that'll give you an extra level 1 spell and a minor cost reduction on your persistent buffs, which can add up.
Skill wise, you want read magic, knowledge (Magic) [This opens up new scrolls for you to scribe, which can be very useful, among other things], concentration, and use magic, in particular. Then anything else that catches your fancy -- play around a bit and see what fits your personal style.
I would almost definitely suggest drow or lizardfolk if you're getting used to playing a caster -- the former has incredibly good line of sight and the latter has built-in armor and innate swimming, which clears up some of the more common terrain issues by itself. Any race will do, actually, and all will do well, but those are probably the ones that give a new player the most initial leeway. Definitely avoid plain elves -- they've got some incredibly nasty weaknesses. A lot of bonuses to offset them, but not something you want to deal with while getting used to the game. Human, dwarf, orc, or halfling would probably be my secondary set of suggestions -- avoid kobolds, gnomes, or, as mentioned, elves. They're not actually weaker or less playable, really, just harder to get used to or better suited for very specific builds (gnome illusionists, ferex).
Your three stat priorities are Intelligence, Constitution, and Wisdom, more or less in that order. After those, a Charisma of 13 (or slightly lower -- within range of a +2 or +3 cha-boosting item is ideal, though) and as high a luck score as you can get is desirable. Strength and dexterity are mostly irrelevant, though the letter is moreso than the former -- dex is good for saves and strength for carrying stuff, but hopefully you'll be play so you can avoid reflex checks, and floating disc covers a lot, if not
all, of your encumbrance issues.
Reasoning in order -- spell failure is the absolute worst thing, and more than
anything that is what you want high int for, to keep from failing when you cast spells. The associated other benefits -- more spells memorizable, more skills, higher DCs for you spells -- are also great.
Constitution should be self-evident -- you don't want to die. Mages actually have a
lot of ways of avoiding being in a situation where you take damage, but when you're learning the more of a buffer you have between you and death, the better. And even once you're familiar with the game, it never hurts to have some leeway for sloppy play or just plain bad luck.
Wisdom is somewhat tertiary, but still important -- it governs your mana pool and thus how long you can last before needing to retreat and recover. You can get by fairly easily with it quite low, but that takes more finesse -- something you want to avoid when you're learning the game.
Charisma of at least 13 lets you barter for buffs, which saves you mana and can get you limited access to druid and priest spells. Luck strongly influences drops, and the more you get of that the more items you'll have, which is good.
School wise, you can't go wrong with Arcana or Weavecraft. Thaumaturgy also gives you more wiggle room in regards to health, but is somewhat less direct in playstyle. Evocations are
very direct, but specialty in it penalizes a lot of very important spell schools -- I wouldn't recommend for a beginner. Necromancy, enchantment, divination, and illusion are all fairly different from your standard playstyle and a little tricky (comparatively), so not something to start with. Abjuration is actually quite straightforward, too, but it's not as easily to lean heavily on a buff-heavy school was it was in the olden days (at least without
abusing scroll casting). Still, perhaps not as relatively finicky as necromancy, enchantment, divination, or illusion.
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To give an example, you'd probably want to start with something that looks kinda' like this:
Race: Drow
Stats (using point buy -- you can almost certainly get better with rolling): 11/8/15/18/15/11/12, coming out to 10/10/15/20/15/13/10 once racial modifiers are factored in. You can actually go lower with strength or dexterity -- but again, as much of a buffer for failure as possible isn't really a bad thing.
Alignment: Neutral Evil -- this gives the most freedom in regards to alignment available, oddly enough. It's only downside is vulnerability to holy weapons -- watch out for paladins!
School: Arcana
Feats: Guildmage, Mystic Preparation
Skills: Two points in: Decipher Script, Knowledge (Magic), Use Magic. One point in: Appraise (this will very quickly help you train your intelligence), Concentration, and Searching.
Spells: Monster Summoning I, Protection from Evil, Floating Disc, Wizard Lock, Expeditious Retreat, and Shield -- though it's possible for the list of available spells to vary. Don't worry about it, too much. Just make sure you get MSI, Floating Disc, and Wizard Lock. You'll start off with Phase Door and Magic Missile.
And you're good to go. First thing you'll want to do is go up the stairs and buy and wear a suit of leather or padded armor, if you've got the gold for it (and you probably will). Don't worry about being untrained -- if the item's description doesn't mention a spell failure rate, there
isn't one, even if you're untrained. Doing this instead of getting Mage Armor will protect you from bonebats, and a few other similar critters. Afterwards, clear out the first dungeon carefully -- don't forget to cast protection from evil and floating disc (to start -- wait for level two to cast anything else, such as shield or expeditious retreat), be sure to use Monster Summoning I regularly, and use Magic Missile to support your summons (just be careful about shooting them!).
Hope that helps! Also, as a fair warning, I was badly sleep deprived while writing that, so if anyone chimes in and offers counterpoints, listen to them well. I've played a
lot of mages, and won with every school at this point, if my memory's not failing me, but I'm also very sleepy, haha!