To better infer the formation and evolution of large structures in the Universe, such as galaxies, it is first crucial to better determine the formation and evolution of stars they contain and more especially increase our knowledge on the star formation activity occurring within their nucleus. Indeed, stars are one of the most powerful "engines" able to drive the past, present and future of galaxies in the Universe. To be more precise, it is particularly important to answer the following key questions to complete our comprehension of star formation activity in the Universe: how stars are forming under various conditions of pressure, temperature, density? How this phenomenon is linked with environments as active as AGN, mergers, or starburst? What are the differences seen and that we can understand in the gas physical and chemical properties in such nucleus? My seminar aims at presenting the potential answers we currently have and develop to these questions. I will present detailed studies of the very dense (10^cm-3) and warm molecular (Tk~300 K) gas, closest component to the star in formation in galaxies. My approach will be both observational and theoretical. I will also present the results I have obtained concerning the study of another gas component - the one responsible of the giant molecular cloud structures in which stars are forming- : the PDR-like gas, less dense and cooler than the previous gas component. Finally I will show recent results I have obtained concerning the comparison between the gas and dust emissions in extragalactic domain.