First, see your doctor to rule out any underlying issues that may be causing difficulty breathing.
Start out at 50/50 pg/vg and see how things go. If you are fine with that, good. If you have issues, try to pinpoint what the problem is - try small bottles of unflavored pg or vg for a bit and see if the issues lessen or increase. You will then have an idea what at least part of the issue is.
It is also possible that you are having a reaction to the flavors, nicotine base, or sweeteners that a particular vendor uses. Different vendors use different suppliers for flavors, nicotine base, pg/vg base, etc. For example, you may find that VapeWild flavors give you issues, whereas Mount Baker Vapor flavors are fine (I am not dissing either vendor, these are just 2 vendors that I have ordered from and am familiar with, I am using them as examples, not complaining about either. We have been happy with what we have received from both vendors, and continue to order from them). With vaping, it is very much a question of how your body reacts, with the equipment that you are using.
Quite a lot of this will be process of elimination, and everything depends on how you react to different aspects of vaping. It may be a simple question of changing the ohms on your coil, or lowering or raising the watts or volts on you mod. It could be that your body is reacting to a component of the flavoring being used.
Once you have an all clear from a doctor that there are no underlying issues, you can, if you choose, start experimenting to see if you can pinpoint what the issue with vaping is. While vaping has helped many people quit smoking/reduce the amount of cigarettes smoked/stay away from resuming smoking after quitting, it is not a be all/end all for everyone. Nothing is. Vaping has helped my daughter and me from resuming smoking after we quit, cold turkey, around 10 years ago, and I am a passionate advocate for vaping. However, I do realize that for some people vaping may not be the answer, due to their body chemistry and how their bodies react to the various components involved in vaping.
I hope that you are able to find a solution to any issues, and stay away from smoking.