Trees in a city give us many benefitss.

=...the city gives us many benefits.

First of all, trees give us a lovely scenery and improve our personal health.

=...improves

They also improve our environment and our quality of life.
They make cities beautiful.
Second, they influence on our emotion and provide many mental benefits.

=Secondly, they influence our emotions...

Healthy forest is essential factor for people to maintain health.

=A healthy forest is an essential factor...

When we are watching trees, we feel calm and peaceful.

=..watching the trees...