Im new at this whole electronic world and i have some questions regarding my first project, a self navigating car. Its a 2WD with geared DC motor i bought from Ebay (The yellow ones you see every where). The car works almost great, it just have a little hard time to take off. The two wheels are connected to a Dual H-Bridge (L293D) where the motors power is coming from a regular 9 voltage battery, now i know thats not much but it should be like 4,5 voltage per motor? Or is the H-Bridge taking up some? Now i dont know if i should be adding another battery to double the voltage or is it the amps that is to low on the 9 volt battery? But my really big question is why does motor 1 go slower if Im forcing motor 2 not to move? If i give both motor a little push they work excellent but as soon one of the motors get stuck when its turning due low power the other one also becomes much slower, whats causing this? Is it some rule in terms of electronics or is it the H-Bridge?
First, a standard 9V battery is indeed too weak in terms of current for such motors. The voltage, however, may be too high (usually such "cars" have 4xAA batteries = 6V). Voltage does not divide between the motors when they are connected in parallel to the battery, as they do in any configuration I can think of.
The problem you encounter is called "stall current" - such motors actually draw more power when a mechanical resistance holds them from rotating freely. Generally, you should find out what the motor's stall current is (you can even measure it, carefully, using a multimeter and a strong battery) and make sure your power source and driver circuit can handle two of these.
In addition to igendel's comments, you should read the datasheet for the L293D carefully (and understand it) - there are a couple of things that might thwart you:
The L293D - being a bipolar h-bridge driver IC - will drop a bit of voltage between the motor and the battery (and release it as heat - so make sure you heatsink the device well).
In addition to #1 - this voltage amount will change depending on the amount of current being drawn by the motor(s).
Both of the above is detailed in the datasheet for the L293D - in short, you will likely find yourself needing to use a battery of a level of voltage greater than your motors - but to know what size battery you will need, you need to do the calculations (and know what the current rating of your motors are - both running (with load) and stall currents).
It might be better for you to purchase a different h-bridge driver if you can - Pololu sells some nice MOSFET based drivers, which won't have the voltage drop issue - but you will still need to know the specs of your motors to figure out which driver is best for your needs.