Configuring VLANs

I need some help in configuring VLANs. I need to seperate two subnets that are running on my LAN. I have a swithed ethernet LAN with primarily HP switches, and one Dell also. All managed switches. I understand the principle behind VLANs, but am not sure how to implement one. My switches have a single backbone, so is it possible to extend the VLAN to multiple switches without seperate backbones? You see, I have my corporate network, with a T1, and I want to put a public access one, with a DSL line, but I have to use the same switches. I have all the documentation from Dell and HP for thw switches, but I’m missing something in the overall comprehension. Can anyone help?

Thanks!
John