is a phrase that behaves in almost exactly the same way as ～のが好（す）き
, but rather than meaning ‘to like’ something, it means ‘to be good at’ something. This particular expression is used with the plain (dictionary) form of verbs only.
She is good at memorizing kanji.
He is good at singing.
In these examples, we can see that there is no difference between Ichidan verbs and Godan verbs. Both will appear in their plain form. In this expression, のが
is performing the same function as in the regular のは・のが
construction. This function is nominalization (turning a phrase into a noun). However, のは
cannot be used with this phrase, as が
is always required with adjectives.
If you want to express that you were good at something (in the past), the verb will still remain in the plain form, but 上手（じょうず） will change to the past tense.
This grammar point can sound a little bit arrogant if you are referring to your own skills, so the word 得意（とくい) may also be used if you want to sound a bit more humble.
I am good at singing. (Like, how am I not famous)
I am good at singing. (I have confidence in my singing)