I’m not good at playing themit, but I practice every day.

You are talking about music and we need to use "it" there and don't forget about comma.

I stopped playing the guitar for several years, but I recently started playing it again.

It is better to complete the sentence with "again" there.

I’m goodbetter at playing the ukulele rather than the guitar.

You need to use the comparative adjective there.

I have a sometimeSometimes, I have a jam session with the harmonica, sometimes I play the ukulele to accompany inthe residents at an elderly care center.

It is better to put the adverb at the beginning of the sentence. Also, with the verb accompany we need to explain whom we are accompanying, and we need to use "at" preposition there.

My favorite singer is kKaze Fujii.

In English we usually remove the double letter like ii. But you don't have to.

Capital letters for names.

I’m not good at playing themmy instruments but I practice every day.

Because you talked about Kaze Fuji between your discussion about your instruments you need to reintroduce the subject. If this sentence was directly after "I love playing the ukulele, guitar and listening to music." then you can say "them" because it's clear.

I stopped playing the guitar for several years, but I recently started playing it again.

Say "again" because it's not the first time.

I’m goodbetter at playing the ukulele rather than the guitar.

I have a sometimes have a jam session with a friend playing the harmonica, sometime. I play the ukulele to accompany them in an elderly care center.

If you say you jam with the harmonica, it sounds like the harmonic plays by itself. :-O

Or do you mean you play the ukelele and harmonica together? If so, then use: "I sometimes play the ukelele and the harmonica together. I play them at an elderly care center."