I've got Skype as well, on my computer and my iPhone. Tons of company's use it so I would imagine it's relatively secure, though I haven't looked into it myself.
You don't need any sort of plan. It's free to use it to chat with other Skype users over the Internet. Video chat's too. The only time you need money for Skype is if you are using Skype to dial a land line. A lot of people use it that way for cheap international calls. But if you need it for a conference call, most likely, everyone on the call will be coming in from the net, and it will be free for you.