It is all do-able, but you will need to roll up your sleeves and start to learn how to do it. The first step is to break down the task into smaller sub-tasks, for example, measuring how much sunlight a single area gets. You will need to need to get a photo sensor, and then calibrate it to your garden, maybe when you read it, you will decide that a reading of at least 100 counts as being in the shade, and 500 is in full sunlight. At the first stage, you would not worry about more than one sensor, battery operation, or it being wireless. Just read the sensor, and write out cloudy/sun/etc. At this stage, you would use the delay function, and take a reading every hour, displaying it on the USB serial device and use the serial monitor on the IDE to keep track of it.
After that, you start thinking about multiple sensors. Initially, I would suggest just using wires to connect each of the sensors, but ultimately you have to thinking about your budget, etc. You would need to think about whether you want several sensors each wireless and with their own battery with a base station that reads each of the values (can be expensive, based on how many sensors you want, and the cost per sensor), or whether it is acceptable to have random wires strung about your garden. I tend to think you ultimately want each sensor to be an independent processor with sensor and battery.
You likely will need to think about perhaps protecting your sensors/microprocessors from rain.
If you are going the wireless route, after getting the sensors, you need to integrate radios, and batteries. Or instead of radios, perhaps use EEPROM during the day to record each hour (or more likely every 5-10 minutes) recording, and then you bring in the sensor at night, to read the values, and recharge the batteries. If you want the sensors to go longer, then you might think about putting a small micro-sd card on each sensor.
Bear in mind, given you are starting from scratch, it will be a lot of work. It might take longer than this season, but you would have it for the next season, and presumably by that time, you would be experienced, and add more things like maybe temp and rain sensors.
Remember Mr. Google and Mr. Bing are your friends, and can often be used to find other people who have done similar projects. For example, this came up when I used google to find weather station arduino: http://blog.makezine.com/2013/01/18/raspi-and-arduino-controlled-weather-station/
And this was found via looking for *arduino monitor sunlight*. http://blog.makezine.com/projects/make-18/garduino-geek-gardening/