probability statistics If there was a tree randomly distributed in a degraded forest at a rate of $5 \text{ trees}/ 1000 m^2$ (meter squared), then what is the probability of finding one or more of these trees in a randomly selected $100 m^2$ plot ??

I have no idea where to start from, can any one help and explain please.
As Michael Hardy said, this is most naturally modeled with a Poisson distribution.

You are told that the rate is 5 per 1000 m2 and asked to find the probability of finding a tree in 100 m2. You need to scale the rate to 0.5 per 100 m2 and then the probability will be 1-Poisson($0;\lambda=0.5$).
This is a Poisson distributed random variable with a mean of 5/1000.

P(equal to or more than 1 trees in 100 m^2 ) = 
1- P(0 trees) = 

1 - (5 x 100/1000)^0 x e^(-5 x 100/1000)/0! = 0.3935