For this proyect you need some knowledge of programing in PHP and MySQL, we will make three archive .php, but this I'll explain in the following steps.
Step 1: You need this
Arduino UNO or similar board ($29.95)
WiFly Shield ($89.95)
Grove - Moisture sensor ($4.99)
Six Jumper Wires ($4.50)
DS18B20 Temperature Sensor with metal tip ($10.59)
If only need, One protoboar ($9.58)
you need a server WEB, with PHP and database MySQL. I used the NAS of QNAP TS-110, is perfect for this project.