Reynolds Number is calculated as: velocity × chord/kinematic viscosity of air
velocity is meters/second, chord is meters, kinematic viscosity of air is around $1.48 × 10^-5$
Then go to Airfoil Tools.
As you make your selections, you can see, in the polar diagrams, the effect Reynolds number has on the lift and drag properties of your wing.
A typical ultra light will have a Reynolds Number around 500,000 to 2 million. A possible path to a solution may be to study aircraft of similar size and speed range. These have been in existence for more than 100 years.
You may find a heavily cambered rectangular wing (Aspect ratio around 6-8) with a blunt rounded leading edge will give good low speed performance and very docile handling characteristics. Variations at this Reynolds range will be mainly in amount of undercamber you choose, depending on speed range you prefer and engine power.
The NACA 4418 may be a good place to start.