Hello Dennis,
You've made a good start by joining this site. We'll help you with your English. Any time you have a question you can come here, and you might find it useful to search through some of the other discussions.
Your English is not bad at all - there are a few minor errors but you are very understandable.
I can correct your message for you:
Hello,
My Name is Dennis and I am 20 years old. I'm from Berlin, Germany. (note that the contraction of 'I am' is 'I'm' - the apostrophe shows that there is a missing letter here and that it is a contraction of two words. You don't still include the 'missing' letter).
I would like to improve my English skills. (English is a proper noun so always starts with a capital letter).
I have great difficulty in speaking and writing English. I can speak a little bit, but cannot understand the meaning of many sentences. I hope you can understand my bad English.