帮忙看看有没有错误,At the beginning,I plan to sing a song.but I never sing English songs.And I don't know which one is suitable for me.So I decide to make a speech at last.Today I want to tell you a story.Now,let's start.

In the beginning. at the beginning后要加of,句中用这个也不太适合.
speech是演讲,后面又说story,好像不太符.
另外,文中都应用过去时.以下是我改的:
At first, I planed to sing a song, but I have never sang in English. And I also didn't know which one to choose. So I decided to give you a story. Now, let's begin.

没有大问题，马马虎虎

开始应该用过去时
At the beginning,I planned to sing a song,but I never sang English songs .除了Today往后，前面的时态要改

Originally I planed to sing a song. However, I have never sung in English before plus I do not know which one is suitable for me. So I decided to make a speech. I want to tell you a story. Now, let's start.

I plan to sing a song.因为你说了是At the begining ,所以应该改成过去式。Iplanned to sing a song.
but I never sing English songs.好像要改成But i have never sung English before.
So I decide to make a speech 同样的，这句话也是发生在过去，所以也应该是过去式。So I decided to make a speech.at last，可以省略咯~~

严格来说,时态有问题
At the begining, my plan was to sing a song, but i've never sang English songs. And i don't know which one is siutable for me. Thus i decide to give a speech in stead. Today, i want to tell you a story.
Now, let's start.

语法没错误.但不知所云.

In the beginning,I plan to sing a song,but I have never sung English songs before,and I don't know which one is suitable for me.In the end,I decide to make a speech.So,today I want to tell you a story...

TODAY前面的内容换为过去式，LET'S START 不是很地道，用HERE WE GO 或LET'S GO 更好