1)By the newest ver. of DF you have to flood the room before you can use it as a farm plot,right?But How can I also use the area where the lake was?I always end up with a big unaccesable hole in the middle of my room =(
I'm not sure I understand what you mean here, but I'll try to explain.
My first underground farms are flooded by building the "farm room" near a "murky pool" and breaching the pool to allow all the water to flood into the "farm room". The water floods the farm, and spreads out far enough that it evaporates, leaving only muddied farm tiles. Then you simply build a wall in the hole you made, so that the murky pool will refill when it rains, and to prevent flying creatures from entering this hole.
My second underground farm works the same way, but I dig out the ceiling above it and rebuild it, so those farms can grow aboveground crops for booze variety and wider textile availability.
WARNING: If you are on a hot or scorching embark, this must be done quickly, before the pools dry up!
WARNING: Don't try this with a stream or brook without understanding how to deal with water.
2)How do you guys start with designing your interior?I often dig gigantic halls for storage but that somehow destroys the fun because I get the feeling that I did something wrong because my rooms are too simple!
It's not the simplicity of rooms that's the issue, it's how efficient that they are. Efficiency, as pertains to construction, is really a function of travel time, and less is better.
For example:
If a dwarf has to walk 20 spaces to a staircase, down three flights, then 30 more spaces to reach an item he needs as an ingredient in a workshop, and he needs three ingredients, this is much less efficient than having a small "input stockpile" near his workshop that holds those ingredients for his use, so that he's taking 6 steps to retrieve all 3 ingredients.
My rooms are fairly simple - I lay my fortress out with a singe central staircase (8 up/down stairs in a square, surrounding an open pit straight to the bottom of the fortress) sitting in a 5x5 room. From this central staircase, a 3-wide corridor goes off in all four major directions. This "cross" forms the major traffic corridors for that floor. Next, I build four 11x11 rooms, one in each "quadrant", and put stockpiles/workshops in these. Stockpiles are specifically tailored to hold only certain things, and are located either below or above the workshops that use their ingredients.
3)What do you guys do with big amount of stones in the earlier phase of the game.
I create a one-tile zone for use as a garbage dump. I dump the stone (d-b-d) and it's moved to this single tile. Then I shut off the zone (and possibly delete it) and reclaim the stone en masse (d-b-c) for use by masons/mechanics/stonecrafters. This takes a LOT of hauling labor, so be careful not to tie your dwarves up too much!
You can also station the dumpsite over the open pit in your central staircase, at which point the stones all end up on one floor, and if the floor is far enough down, any miasma from rotting items won't reach your dwarves on upper floors. Think in terms of material flow, rather than in terms of items.
4)Is the game too easy?When I start the game in a neutral region I never get attacked and I always find lots of valuable things like gold or platinum too soon (with large amount of beasts and savagery)!!!!!!!But in evil regions I loose like in 10 minutes because some random zombie guerillas come and kill me!Am I doing something wrong?
If you're surviving sieges and ambushes and hostile wildlife in the wilder neutral territories, and you find it too easy, then you can try evil zones, but the catch to evil zones is getting below ground ASAP with as much as you can salvage from the wagon, then walling yourself in to stay safe while you get ready for war!
Generally, however, if you find the game too easy, you need to start setting more challenging conditions for yourself, like embarking on glaciers, all-mountain biome embarks, limited skill or item embarks, increasing evil/savagery, or seizing goods and killing merchants to make elves and humans go to war with you (which can take a while). The wiki has lots of "challenge" embark ideas. Try those.