Monitoring Network for Public Access

We have a library with internet access, nothing fancy, plain old internet from an internet provider and a couple of extenders around our space.

Our worry is that since we have no idea what people might be doing on our network, it could be a threat.

1- we do not want someone to take advantage of our network and perform illegal activities

2- we do not want someone to use this public network to spy on other users

How can we combat these threats?