how to start developing a data mining tool

Please forgive me for saying, but you can start by putting extra focus on correct English spelling.
You will find that, with correct English spelling, people are more likely to help you.

One of the unique selling points of C#, is the integration with data-bases and SQL.
You might want to make a selection of your favorite learners, e.g. decision trees, neural networks, etc.
Then you can find the C# implementations, and combine them with a good visualization library.

Data manipulation (SQL, etc), modelling (decision tree, etc), and visualization (plots, etc) are the corner stones of any data mining toolkit

Best regards,

Wessel

thank you for the advice but english is not my mother language and i will try to avoid mistakes.
and to the proiect iam going to use decision trees because the tool will be for classification .
thank you again

No offense on your intention to write a data mining tool, but instead of reinventing the wheel, why not base your work on an existing and mature piece of software and extend it with your operators? RapidMiner provides you with all the infrastructure you need, including loading, manipulating, visualizing and storing your data. instead of reimplementing all this from scratch in a new tool, you could base your work on RapidMiner and focus on the real interesting stuff like implementing new algorithms, your cool decision tree etc.

Just my 2 cents.

All the best and good luck with your work,

Marius