As midders stated, you will need to install DSL filters between every voice telephone (this includes landline telephones with cordless handsets and answering machines) and the main telephone line which carries both voice and ADSL signals. (For example, if you have three telephones connected directly to the same main phone line, you will need three DSL filters. The exception is if you have one cordless telephone base with multiple cordless handsets using it, then you will need only one filter - between the base and the line.) If one of your telephones is connected directly to an answering machine which is then connected to the phone line, then you will only need one filter between the answering machine and the phone line for that location (no filter is needed between the phone and the answering machine). The only device which may be connected directly to your telephone line without a filter is the DSL modem itself. If your modem does not include the filters, you will have to contact an authorized reseller of AT&T DSL broadband service to find out where you can purchase the DSL filters.
In addition, if your ADSL modem has only one Ethernet connection, it means that it is nominally designed to be used with only the computer system that's hard-wired to the modem. If you want to connect more computer systems online through the same ADSL connection, you may want to add a wired/wireless router to the setup. (The reason why I said "wired/wireless" is that so-called "wireless" routers actually have four Ethernet LAN connections for up to four hard-wired computer systems or video gaming systems or networkable Blu-Ray players plus one Ethernet WAN connection for the external modem or other network connection in addition to wireless connections to over a hundred wireless Wi-Fi devices.)