。
My hobby is collecting old coins.
Literal
I [of-の] hobby [topic-は] old coin [object-を] collect thing is.
The X は Y ことです structure ('X is [the act of] Y-ing') is the standard way to define one's hobby in Japanese — the こと nominalizes the verb so it can serve as a predicate noun. 集める ('to collect') is the transitive verb; 集まる ('to gather, come together') is its intransitive pair — a common Japanese verb pattern of paired transitivity. Old-coin collecting (古銭収集) is a longstanding niche hobby in Japan, with dedicated shops and clubs.