designing a poster including 3 activities of mu office

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my office works in relation with about five programmes that are education, culture,ecology,hydrology and basic science. there is a celebration that we want to highlight three activites of three programmes from the previously mentiones programmes. these activites are culture, ecology and hydrology. we want to present them in a unique poster. i have some ideas and have the material to use, but i need more details about making a poster on the pc, like which software to use and what technique should i follow and also information about how to print it in the best resolution