You can start making basic "redstone" devices as soon as you have enough wood to make a door and a pressure plate: make the door and the pressure plate, and put them next to each other. Voila! Your first "redstone" device without needing any redstone.
If you want to start getting into more complex devices like flip flops, you need redstone dust: you can find it by mining redstone ore deep underground: 16 layers above bedrock or lower (sea level is at the 64th layer).
The best way to learn how to make redstone devices like flip-flops is to watch the myriad of tutorials on YouTube: figure out what you want to make, search for a tutorial on how to make it, and repeat it1.
For example, Rolf-David's tutorial shows you how to build a basic T Flip Flop:
The basic flip flops (and most redstone devices) are pretty simple, but aren't very efficient because of the way redstone works: they take up a lot of space and because power doesn't travel instantly across redstone, larger circuits are incredibly slow.
So what people have been doing is exploiting buggy implementations of blocks not intended for redstone circuitry to make them smaller and faster. Etho is probably best known for exploiting what's called the "block update"; a quirk of Minecraft where blocks send power when they update their status:
For the most part, if you never got into advanced features like this, you'd still be fine.
Note 1If you want a more comprehensive education on Redstone, there are couple of channels that focus on it pretty heavily: Etho's Lab and SethBling probably the two biggest ones. Etho's Lab is more "Let's Play"-ish (although he does do stand-alone tutorials from time to time), whereas SethBling is pretty heavily focused on tutorials.
How many solar panels you need is up to you. Even though Forestry farms drain a lot of power, they're A-OK with receiving very little, and in fact I'd recommend that because you get seriously diminishing returns if you power them fully.
The wheat farms especially, because it only has to harvest the crops and replant. Other farms have to do a lot more (such as harvest multi-blocks structures, replace soil blocks, gather loose items). The wheat grows on its own, and you're probably OK with the farm and harvester taking a bit to harvest and replant.
On Forestry easy mode, I'd say 1-2 MJ/t is enough between the two machines to keep things going, which means a single electrical engine (Choke upgrade or not). You don't have to alternate, a forked conductive pipe will do the trick. I recommend giving the electrical engine double its intake so it can run through the night (obviously you're gonna need a buffer like a BatBox as well). Maybe a bit more so it can cope with inclement weather.
Let's say you have that engine set up with Choke and Efficiency, which means 3 EU/t. Use 3 solar panels to have it running during the day, 6 and a buffer to have it run through the night as well, and maybe 7-8 if you don't want rain to slow it down too much. (Although day is a bit longer than night, so it should slowly build up a buffer even with 6 solar panels.)
Best Answer
Two options:
Use a Detector Cable. It can accept up to 32 EU/p and emits a Redstone signal if energy is flowing through it. However, it has a very high energy loss (0.5 EU per block).
Use the BatBox or equivalent storage unit itself. If you right-click it, you should have a button with Redstone dust on it. Click that until you get the "emit if empty" behavior. This will make the BatBox output a Redstone signal if it doesn't have energy to send. It's smart enough to output a signal if it has some leftover energy, but not enough to send a packet.