data={ {0.25,1.517}, {0.36,1.665}, {0.49,1.798},{0.64,1.925}, {0.81,2.038}, {1.0,2.150}, {1.21,2.260}, {1.44,2.359}, {1.69,2.464}, {1.96,2.545}, {2.25,2.634}, {2.56,2.728}, {2.89,2.817}, {3.24,2.885}, {3.61,2.979},{4.0,3.038}}

original=ListPlot[data,Frame -> True,
                                             FrameLabel -> {"Cuadrado longitud (m2)","Periodo (s)"},
				  ImageSize->Large,
                                       PlotStyle -> {PointSize[0.02]}]

model=LinearModelFit[data,x,x]

Plot[model["BestFit"], {x, 0, 4}]

Show[ListPlot[data], Plot[model["BestFit"], {x, 0, 4}]]