at89c51 programer
Why did you choose AT89C51?
Do you have a programmer to program it?
I don't think so, so better to start with microcontroller (also based on 8051-core) which can be programmed through their serial port, without external devices.
You can choose one like that from, for example,
www.maxim-IC.com .
Other thing is to select proper A/D converter. Again, you can do it at Maxim, or other places..
It looks like for your purpose you don't need anything above 12-bit, and probably 10-bit A/D converter would be sufficient.
After you decide what exactly you would like to measure you can start selecting proper sensors, so let us leave it for a while..
As far as software is concerned, I would encourage you to start with assembly. This approach will give you full in-depth understanding on how a microprocessor works and how to control its internal/external memory, registers, inputs and outputs ..
There are plenty of links on the 8051 assembly (search this forum first), including thousands of examples, and I am pretty sure many of EDA members will give you plenty of valuable advices