Before you can even be able to work on any kind of HVAC system you will have to have the proper HVAC training, but before you can get down to learn how to troubleshoot you should to be epa 608 certified. This is a certification that is required by the EPA, and if you are to handle any kind of refrigerant you must be certified. The penalties for not having this license and handling refrigerant can be pretty hefty. So no matter how much HVAC training you have, or how much you know about troubleshooting air conditioning systems, you must have your epa 608 certification. This is the first and most important part of HVAC training.
Now the purpose of getting this certification, is not to actually give you hvac training on troubleshooting air conditioning and heating systems. It is for the safety of not only you, but also the ozone layer, as there are many kinds of refrigerant, also known as freon that can destry the ozone layer. This is critical as this is the only earth we have and we have to take care of it. This is why having your epa 608 license is so critical when it comes to hvac training.
Now there are 4 types of certification when it comes to the epa side of hvac training, the four parts consist of what types of systems you are able to work on, but there actually only 3 different tests you have to pass. You have a type 1,2,3 and universal hvac license. The first type of hvac certification is for handling high pressure systems, such as window air conditioners. The second type of certification consists of working on residential air conditioning and heating systems, which is the most common type for many people. The last of the hvac training, epa certifications is type 3, which is for low pressure refrigerant systems.
High pressure refrigerant systems consist of commercial and residential air condtioners. If you have all three of these types of certification then you will get a universal license. So unless you have this, or the particular license that you will need, do not work on any kind of hvac system if it means you will have to handle refrigerant.
Now if you wanted to do some maintenance, like clean the coils, change the filter, unclog the drain line, then by all means this type you can do because you are not handling any refrigerant, or freon. When it comes to other stuff though like charging a unit, recovering freon, or even touching a freon container, you better have the proper hvac training, you better be certified.
Update: A comment was brought to my attention that I was giving out false information which I was and it has been edited. So just to clear things up this is straight from the source.
608 type 1 - (small appliances less than 5 pounds of refrigerant)
608 type 2 - (high and very high pressure appliances)
608 type 3 - (low pressure appliances)