The LaTeX command for contour integral (∮) is \oint (line integral over closed curve).
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Write \oint inside math mode, that is between $…$ for inline math or in a \[ … \] or equation block for display math. It is part of core LaTeX math, so no extra package is required.
In display math \oint takes its full size; inline it shrinks to fit the line, and \displaystyle forces the large form. With unicode-math under XeLaTeX or LuaLaTeX, typing ∮ in a formula gives the same symbol.
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Use \oint in math mode; it needs no extra package. For example, $\oint f(x)$.
None: \oint is built into LaTeX, so no extra package is required.