fittin a regression line

Dear sirs,

I have tried to find a command to fit(plot) a regression line but I
haven't found any command on the html.help. 
My question is if I am using a regression for instance reg<-lm(y~x) and
I would like to make a regression line, to see how well my data is fit.
Can you please help me with this??
Yours Sincerely
Soheila Zad
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