In this talk I will address the problem of determining stellar parameters in young objects and measuring their magnetic activity level, which are strictly related. The photospheric/chromospheric activity produced by the stellar magnetic fields has indeed a strong impact, together with the mass accretion from circumstellar disks (when present), on the determination of stellar parameters. I will focus on my experience in these topics, which is mostly based on the analysis of medium-resolution (GIRAFFE, X-Shooter, LAMOST) or high-resolution (ESPRESSO, UVES, HARPS, etc.) spectra of YSOs in young clusters or star-forming regions in the framework of large surveys (such as the GES) or dedicated projects (e.g. PENELLOPE) for which I generally used the ROTFIT code developed by me. In particular, I will show how it is relevant to include the determination of the veiling and the treatment of the rotation velocity, together with an adequate choice of the models and spectral regions to be analyzed, both for the determination of the stellar parameters and for the measurement of the indicators of chromospheric activity by means of an appropriate removal of the photospheric spectrum.