about VPN solution

[vladone]

Hi!
I have an private network, that acces the internet via an freebsd
gateway. I want to buil some authentication for my users, to prevent
ilegal connections. When an user want to connect to my gateway (to
acces the internet), require to enter user and password.
My questions is:
What solution, is best for this?

I read some about VPN, but if someone have an better documentation
aplicable for this situation please!