Firstly, please note that Video Chats can only be started by members who are Premium or above, however anyone on RedHotPie can accept a video chat.
If you are able to initiate video chats then you can click on the video camera icon on the top-right of a Conversation. Whether the other party will accept a video chat is dependent on their own personal settings. By default, you are set to accept video calls from 'Anyone' but you can adjust this setting at any time by clicking on the cog icon on the top-left and changing settings as needed.
If the other party has opted to only receive video chats from people who match who they are seeking, or from people they are Friends with or have Fav'ed, or they have chosen to not receive any video chats at all, then you will not be able to start a video chat with them.
If the other party has opted to receive video chats from 'Anyone', or you satisfy the criteria according to their video chat settings, then they will be notified of an incoming call. If they accept the call then the video chat will start.
Both parties can choose to video chat, or just one party can show off on cam, or you can both opt to have a voice chat only.