It all depends.
Hardware:
Laserscanners can be connected over a serial connection or USB or Ethernet, depending on the type of the scanner, IMUs are often connected over Serial or USB, low level sensors like switches or wheel encoders are often connected to a microcontroller which is connected to the main PC.
Software:
You need ROS-drivers for the Sensors, for some laserscanners and IMUs you can find ROS packages, or you can program them yourself.
Some motorcontrollers have inputs for wheel-encoders.
And now for something completely different:
A IMU alone is not able to measure a position in space, and to assemble a 3D scan from a flying not really known position will not be easy. Why don't you try to generate a 3D scan from a fixed position first.
Edit:
for a non flying test: I meant keep the position fixed, but change the orientation of the scanner (tilt it or rotate it), then you don't start with both problems of measuring the orientation and the position at the same time.
Do you know laser_assembler ? This could start you on the software side.
How are these sensors connected to your computer? What kind of robot are you using?
That is what the question is, how should i connect those sensors to computer. Have i to use usb serial communication with every sensor to get its data into computer ? And then after getting into the computer whats the way to get this data into Ros
I have my own quadrotor, i am not using any specific ros robots