How can I communicate at a airport eith many ßlanes, but with no ATC or just Atis?

What should I do if I want to take off at a bussy airport, that has no ATC but only atis or not even that.

Use Unicom as a rule, though the USA is conducting a trial of using CTAF instead currently, so in the USA use CTAF.

also worth noting: if there is an ATIS for an airport, somebody is covering it
there will be cases in which the airport is uncontrolled and you coordinate with other traffic on unicom (or if in the US: on CTAF, as Don says) regardless, but if an airport is controlled, you will have to call the responsible controller and get the necessary clearances from them