Since you are learning Chinese, it is best that you also provide an English version.  What you wrote is open to a thousand and one way of correcting.

 

两只老虎

我学会了一首关于两只老虎 的詩. 这是真很不容易.  I had learnt a poem about two tigers.  Really not easy.  I corrected this way because 首 is a classifier for poems, songs, lyrics etc.  You had left out the object to the sentence.  So we are guessing as such from the use of the classifier.

(Wǒ xuéhuìle yī shǒu guānyú liǎng zhī lǎohǔ
zhè shì bù róngyì)

两只老虎

我学会了一首关于两只老虎 的歌曲(I have learnt a  song about two tigers.)
这是不容易 （这是不容易的事。 这很不容易。these two sentences are more authentic.）
(Wǒ xuéhuìle yī shǒu guānyú liǎng zhī lǎohǔ de gēqǔ
zhè shì(hěn)bù róngyì  de shì)

两只老虎

我学会了一首关于<em>名字叫做</em>两只老虎<em>的</em><em>歌曲</em>

这是<em>很</em>不容易