Any recommendations for any sites or links to help me to pronounce german correctly ?
I'd start with this playlist for learning the sounds. You can also watch other videos from this channel and similar channels to improve your knowledge of german.
However, the best way to improve is just to practice. So maybe you start with a sentence from duo you struggle to pronounce and you just say it slowly enough times until your mouth gets used to doing certain shapes. You can practice wherever and whenever just whispering to yourself whatever phrase or sentence you're practicing. With time, you'll get better and Id also suggest reading out loud the sentences and words you see in your normal duolingo lessons and practices.
You can also make use of other services, Memrise, for example, has all the vocabulary read aloud by a native german speaker and you could copy the sounds it makes as you practice yknow?
To have a bit of fun with it, take anything from the playlist that Islacorn mentioned or get the text and sound file from any of the Deutsche Welle langsam gesprochene nachrichten casts and run through them once or twice to get a feel for the sounds.
Then install something like Audacity and record yourself saying the sentences that the German natives are speaking. That way, with playback, you'll hear how you are sounding versus the native speaker and can adjust as necessary.
Not sure about you, but I find many of the "sch, sh, s, z" sounds the most problematic to pronounce. I sometimes look for combinations and try them out in my mouth. "Blitzschnell" is one good word to work on. "Schlafenzimmer" is another one. Many more out there. Und niemals hat ein Meister vom Himmel gefallen. Practice makes perfect.