Okay so, i am lvl 25, and i only have 140 mana..... 4/5 of the spells that my skills are high enough for take like double my mana or more! How do i get more mana? I mean have a spell that costs 1400 mana!!1 Yes! thats not an extra zero by mistake, its actually 1 thousand, four-hundred.

Ok, first of all, it's 'magicka' not 'mana'.
Second of all, you increase your magicka with enchantments, potions, or spells (ironic eh?) and you can also increase your magicka by increasing your intelligence when you level up, or by fortifying it with potions, enchantments or spells.
Third of all, the cost (in magicka) of a spell decreases as you level up in the skill that governs the spell you are trying to cast (I've provided an example below).

Example: a person who's level 1 in destruction is attempting to cast a fireball spell that does 256 damage over an area of 100 ft., but he fails because the spell costs 780 magicka and he only has 300 magicka. A person who's level 100 in destruction is attempting to cast the same spell, and succeeds because it only costs 170 magicka points for him to cast the spell.

hope this helps