Digital speedometer

hi
i have an idea to build digital speedometer with distance travel in kilometer/meter ,show the speed (km/h) and to find how much distance total distance.
need to press push button if wanna to display other measurement.

i want to use 16x2 lcd, 8051 cip,
and Photomicrosensor(example: Gap Slotted Optical Switch) or hall-effect sensor.

I want to use it in my Bicycle and further in my bike.
sensor should be easy to install and simple too
How to start?

Personally I'd use a PIC microcontroller.

First thing to do is to get whatever controller you're using working with the LCD. This is the hardest part of the project. Get it talking to it, have some dummy code in so you can simulate the mph/kph, get the layout on the screen correct then work on the code for the distance and speed (fairly easy).

You haven't actually given any information as to the level of your electronics experience.

It would be far cheaper (but less fun) to buy a $10 unit designed for a bike which does all the above, has very low power consumption and is already waterproof.