Stars can strongly interact with their close-in planets through their magnetic field. The stellar magnetic field is the driver of activity in the star and can trigger energetic flares, coronal mass ejections and ionized wind. These phenomena may have an important impact on the magnetosphere and atmosphere of the orbiting planets. In this project for Foreign Student Program, we will focus on how stellar magnetic fields, their winds and flares impact close-in planets. Also, we will study how the magnetic reconnection between the planet and the star can trigger stellar activity. To accomplish that, we characterize spots (radius, intensity, and position) on the surface of some stars by fitting the small variations in the light curve of a star caused by the occultation of a spot during a planetary transit. Next, we develop stellar magnetic maps using the spots distribution on the stellar surface. From the spot temperatures we can determine its magnetic field intensity using the same relation of sunspots. This magnetic configuration is used as input for the three-dimensional magnetohydrodynamics numerical simulation of the stellar magnetic field. Moreover, the stellar interaction with a magnetized planet is investigated.