Hi Edmond
Have you set up goal tracking yet?
If you haven't, set up a goal so that you can track leads from within GA.
Once you've done this, do the following:
1. Acquisition > Channels
2. Under the conversions section, select the goal you wish to track
Once you've done this, you'll be able to easily see which channels have led to conversions, what the conversion rate for each channel was etc. You can also find an even more simple report from the 'overview' tab in 'acquisition'.
Once you've got goal tracking established, you can make better use of the entire acquisition section, as well as segments etc.
I think this is the most simple way to do it. From there, you can drill into individual channels for more data about where your leads are coming from.
Hope that helps!