What I have learned about designing aircraft in KSP....
First of all throw out just about everything you may know about aircraft design, most of it doesn't apply here.
Always try to put your center of lift on or close to your center of gravity (this makes the aircraft far more stable and control-ably agile in the air)
Canards are the spawn of satan, particularly on small craft (unless you really like flipping over and over and over out of control most of the time)
Learn all the controls for building, particularly the one that rotates parts by 5 degree notches (hint it involves the shift key)
Don't be afraid to layer your wings right on top of each other, the game doesn't care about aerodynamics much (I actually ignored this craft for about 15 minutes and it was still flying on it's own, just was doing slow loops over and over and over)
landing gear can be a royal pain (particularly with one size, small) and can cause your aircraft to skitter around/crash on the runway during takeoff/landing if placed wrong. It is also difficult to place them without making it very likely you will tail-slap the runway
Ailerons are useless to super dangerous. They tend to interfere with climbing and diving, and make your plane hyper sensitive in rolls as the game does not separate the two and will use ailerons to climb and elevators to roll (for example with this plane, which I use as an example of a well built plane below, I could not get off the ground once I put on ailerons).
Fuel drains from front to back, which can seriously unbalance your aircraft as you fly. You pretty much have to manually transfer fuel forward in flight, or design your craft so that the fuel is mostly in line and drains inward using pipes
This for example would be a well designed plane, it is very stable and balanced and has good control characteristics.
It is made out of:
1 mk1 cockpit
4 mk1 fuselage
3 small gear bay
2 delta wing
2 structural pylon
2 xm-g50 radial air intake
3 av-r8 winglet
So why would you want to use planes and not rockets? Well they are incredibly fuel efficient, and very cheap to use and they can be made 100% reusable.
Downsides though is that the selection of stock parts is pathetic (get mods), and these are the most difficult to design type of craft in the game.