really, it would be great if you could apprentice under an experienced consultant, to learn the ropes.  However, re-reading your post again, I realize you work for this farmer on weekends and during the summer.  If you go apprentice under an experienced consultant, you will not have the opportunity.

Look at it this way... the fact that this farmer has offered you this opportunity tells me that he has a lot of confidence in you.

When I started scouting, I mentored under our local, very experienced County Agent.  That experience was very valuable to me. I was in his office every week, sometimes more than once every week - asking questions, bringing in weeds, bugs, plants with symptoms, whatever I was finding......   asking questions, and solicting his advice.    As I became more experienced, I didn't need to spend as much time in his office.  The flip side of this:  I was also a source of information for this county agent.  By bringing in samples of what I was seeing, this county agent got to see what was going on in the field.  That was valuable to him as well.

I have these suggestions:

a) today, if you have not done so already, start accumulating some resources: weed ID resources, insect ID resources, disease ID resources - whether they be printed material, or know where to find them on the web.

b) start making a list of basic problems to look for, and when.   Start to notice when particular weeds germinate in your area, when particular insects of interest start to show up in your area,  etc.  Knowing when to look is as important as knowing what to look for.

c) I concur with Ed.  Find yourself a mentor.  Someone you can go to with problems you find, questions you can ask.  In this day and age, that might even be someone you know on the internet. With camera phones, you now can take a pic, and e-mail it to your mentor, asking "what am I seeing here?"  Remember, if you are seeing a problem in one field, that might be a good sign to check all your fields for that problem/ pest.

d) Check to see if your state extension university offers a scout school. Our university started offering one in recent years.  If available, consider attending. 

Good luck.  Remember: show up, be observant, ask questions.......