Patents aren't that complicated to get. Although, a patent lawyer or patent agent (cheaper) is usually suggested, you could potentially do it without one.
You need a document that lists:
1) Your claims. That is, what specifically are you claiming to be new and non-obvious. If you are patenting a pencil with a different color eraser, that probably wouldn't be accepted. Why? Colored erasers are obvious changes. If you are patenting a pen with an eraser, that wouldn't be accepted. Why? Pens with erasers are not new. Make your claims specific and don't leave out anything. This is what you get patent protection for. The rest of the patent is helpful, but you are only given patent protections on what you specifically claim. Aim for ~50 claims if you can. About half will be rejected and if you have more than 25 accepted claims, the costs start to soar.
2) A description of the invention. A patent is a deal. You tell the world how to make your invention, and in return you get the rights to exclude others from your invention for a set period of time. Thus, a patent requires you to describe your invention in detail. It must be clear enough that a collegue can understand it and build it himself/herself. Use text, and even better a numbered diagram. Then say how everything connects together. You can't say a human body has two feet, two arms, and a head. Why? Because that doesn't say how they connect. You need to say the foot bone is connected to the leg bone, the leg bone is connected to the knee bone, the knee bone is...
3) A few background bits of information are needed. What field is this patent in? What similar inventions are already available? How is your invention different/better.
Download a few patents in the same area as your invention and look at them. You'll get a good idea how to write one.
If you do that all yourself, a patent lawyer/agent can rubber stamp it and send it off for minimal expense. Or, you can pay them to write the whole thing ($200+ an hour for a good lawyer will add up into many thousands of dollars). Then send it off with patent examination fees. Oh and you'll have yearly maintanence fees. Overall expect $10,000 if you do it mostly on your own or $15,000-$20,000 if you use a lawyer.