The first sentence needs more context. Does the sentence end there? Or are you trying to say "I have improved..." followed by a noun?

If all you're saying is, "I have improved." in the sense, "I have gotten better," then

"Je me suis amélioré(e)."

is perfectly fine.

The verb "s'améliorer" is reflexive, which is why Google Translate used "être" for the auxiliary in the past tense.

Otherwise, "to improve (something)," or "améliorer (quelque chose)" in French, follows transitive verb rules in French, which means you would use "avoir" as your auxiliary in the past tense.

So if you just use "avoir" in place of "être" in the second sentence that Google Translate provided, it would work fine:

"Je n'ai pas seulement amélioré mes compétences..."

The first sentence needs more context. Does the sentence end there? Or are you trying to say "I have improved..." followed by a noun?

If all you're saying is, "I have improved." in the sense, "I have gotten better," then

"Je me suis amélioré(e)."

is perfectly fine.

The verb "s'améliorer" is reflexive, which is why Google Translate used "être" for the auxiliary in the past tense.

Otherwise, "to improve (something)," or "améliorer (quelque chose)" in French, follows transitive verb rules in French, which means you would use "avoir" as your auxiliary in the past tense.

So if you just use "avoir" in place of "être" in the second sentence that Google Translate provided, it would work fine:

"Je n'ai pas seulement amélioré mes compétences..."

If you're looking for sites that give you more examples and have their own forums, I use WordReference.