data={{0.5,1.517},{0.6,1.665},{0.7,1.798},{0.8,1.925},{0.9,2.038},{1.0,2.150},{1.1,2.260},{1.2,2.359},{1.3,2.464},{1.4,2.545},{1.5,2.634},{1.6,2.728},{1.7,2.817},{1.8,2.885},{1.9,2.979},{2.0,3.038}}

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

model=LinearModelFit[data,x,x]

Plot[model["BestFit"], {x, 0, 2}]

Show[ListPlot[data], Plot[model["BestFit"], {x, 0, 2}]]