It's "a pair of tongs". This object consists of two parts; therefore, it only has a plural form.
It's "a pair of tongs". "Tongs" is a plural noun and only has a plural form.
We need to buy a pair of tongs.
We need to buy a tongs.
Since the article "a" is for one, you cannot use it before "tongs"; however you can add other determiners to identify or quantify the noun, including "a pair of", "the", "some", "these", "several", "both", etc.
A pair of tongs can be used to place and move hot coals.
Both tongs are very versatile.
You can also say "two pairs of tongs", "three pairs of tongs", "four pairs of tongs", etc.
She used two pairs of tongs to cook.
When using this term in a general sense, omit the determiner.
Tongs commonly have flat ends to help pick up items without damaging them.
Keep in mind that this word requires a plural form of a verb. We say, for example, "tongs are/have" (not "is/has").
These tongs are too expensive.
This tongs is too expensive.
A partial list of tool names that also consist of two parts, and therefore do not take the article "a":