Each study enrolls patients with certain types and stages of cancer and certain health status. A study that involves two or more treatments can yield reliable answers only if all the patient cases are the same so they can be compared with each other. Staging tells how far the disease has spread. Deciding on treatment depends on many things, including the stage of the disease and your general health. You would most likely be referred to a trial by your own doctor or by a doctor who knows your case.
Some patients find out about trials from other sources.
In any case, you must have a reasonable understanding of your role in a research study and be freely willing to take part in it. Ask what you can expect if you take part in a trial.