First, you must know how many valence electrons each atoms requires to fill its outer shell. Since hydrogen is a first row element, it only requires 2 electrons to fill its outer shell. Meanwhile, chlorine is a second row element and requires 8 electrons to fill its outer shell.
There is a single bond between hydrogen and chlorine in H-Cl, and that bond corresponds to 2 shared electrons. Those 2 electrons are enough to fill the outer shell of hydrogen, and also count towards the 8 required for chlorines octet to be full. Therefore, chlorine requires 6 more valence electrons to complete its shell, and those are shown as lone pair electrons around the chlorine atom as drawn below:
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H:Cl:
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