I have many ideas for electronic projects that are controlled by a Personal computer. my question is where can i get Input sensors (light, speed, proximity, temperature, current etc...) and output deviecs (Relays, motordrives etc...) that can be connected to my computer, and can be controlled and processed directly by a program I write (I program in C/C++) not by a program that comes with the product...
You're generally going to want to operate off the RS232 serial port. To get programmable outputs and sensor inputs, generally you will use a PIC microcontroller with a simple MAX232 interface chip to turn RS232 into 0-5V logic. There are many, many PIC projects documented on the web which have done such things. A good PIC programmer costs under $100 and individual chips run $2-$6.
There are several C compilers for the PIC.
Since you're going to be putting code on a microcontroller, consider if you can just put everything you want to do on the microcontroller and free it from the PC.