allanlim: I am a newbie to this game. How should I allocate the mana? What percentage for research, casting skill? Any advice?
As EvilLoynis says, there's no "one size fits all" rule. But for a beginner, these general rules of thumb should suffice.
1) Make sure the mana coming in exceeds the mana going out. In other words, if you cast a spell (such as a summon) which has a per-turn mana cost, and you run out of mana, that spell or creature will be automatically dispelled. On the right side of the main screen, mana is the third number (under gold and food); like gold and food, if it turns red (-1 or below) you have a problem.
2) I recommend keeping research at exactly 5 or a multiple of 5, at least at first. As you build buildings that help with research (libraries, shrines, etc.) adjust the slider accordingly. This is because research points not used in a turn are wasted, but all spells have a research cost divisible by 10 (and therefore also by 5). Research the lowest-cost spells first, unless you see one that you just have to have. The game will show you how many rounds it'll take at your current research level.
3) The Skill slider takes the longest to build, and also the longest to see the benefits. One school of thought says to wait till you have the points to spare; the other says to start growing it right away. Skill determines how much mana you can cast
per turn, both overland and in a combat; low skill means that the better combat spells are inaccessible, and even the weaker ones are severely limited in how often you can use them.
4) In an emergency you can burn gold to make mana, and vice versa, by way of the Alchemy button on the Magic screen. You lose half of whatever you're converting, unless you took the Alchemist retort as part of your build, so do keep that in mind.
Specific builds come with their own variations and exceptions to the above, but for a first-timer these should at least get you headed in the right direction. Have fun and good luck.