I'm combining the other two answers and adding a few things. : )

Starting to learn English

Hello, I am Duc, I'm Vietnamese, i I am 18 years old. I want to learn English, let's so help me, please.
And, I can help you learn Vietnamese if you want :) Thanks very much :)

For the second sentance, you can use any of these:

"so help me, please."

"so please help me."

"will you help me?"

"can you help me?"  (This is kinda wrong but Americans say it anyway)

Start learn English

Hello, I am Duc, I'm Vietnamese, I am 18 year old. I want to learn English. let's Will you help me, please?
And, I can help you learn Vietnamese if you want :) Thanks very much :)

*In English (in general), you conjugate the first verb, and leave the second one attached to "to"
ex: I want to eat. I like to swim. I run to win. 

Starting to learn English

Hello, I am Duc, I'm Vietnamese, I am 18 years old. I want to learn English, let's help me, please.
And, I help you learn Vietnamese if you want :) Thanks very much :)

Great! I'm not a Native English speaker, but I think you're writing very well. Just some mistakes... 

That's awesome Duc. I'm Matheus and I'm from Brazil. If you would like to learn Portuguese one day or just practice English,  I can help you too haha.