Tips on pronunciation
I recently went back (again) to Norway and noticed that while I could read some sentences quite easily, a lot of people could not understand my pronunciation of norwegian and would switch to English in 80% of the cases rather than answering me in Norwegian. I also actually noticed I had difficulties understand basic sentences such as "which tea do you want" even though I am doing the listening exercises very regularly on duo.
Any one has tips or sources to improve pronunciation?
The best way to get better at speaking and listening is to speak and listen. :)
NRK has plenty of TV and radio shows available, and you can check out this thread for more resources. This way you'll be exposed to a variety of dialects. Also make sure you read every exercise here on Duo out loud - not just the ones that ask you to do so.
Norwegians will switch to English at the slightest sign of struggle, and generally do so as a (misguided) courtesy to you. However, you can always tell them that you're learning and would prefer to continue in Norwegian. Unless they're busy, I'm sure they'll be happy to oblige. A trick is to approach older people, as they tend to be less comfortable with English and often have plenty of patience - not to mention the best stories to tell.
Chances are pretty big that someone has talked to you in another dialect than the eastern Norwegian that is closest to Bokmål. After all, most of the population speaks some other dialect.
Spend some time listening to, and getting used to the different big dialect groups, primarily Nordnorsk, Trøndsk, Vestlandsk and Østnorsk. You will of course never need to bother learning how to speak any other dialect, but it is necessary to learn how to recognise the main groups, in order to understand people well. It doesn't matter very much if you don't plan to travel around much in Norway, because in the cities you will meet people from all over the country, speaking their own dialect at all times and in all situations. Luckily, this also means that different dialects pop up all the time if you watch movies, videos, new, listen to radio, etc. There are also music artists singing in all kinds of dialects.
Look up all the consonant and vowel sounds (phonemes) there are in Norwegian (do this on Wikipedia for example). Then find a word for every sound and learn to pronounce it as accurately as is needed for the sound to be distinguished from every other Norwegian phoneme.
This is a good basis for getting a good understanding of Norwegian pronunciation and will ensure that you will never have the problem that many language learners have: you won't merge any sounds and will be able to tell them all apart.
For example: å sy /syː/ vs. å si /siː/ vs. ski /ɕiː/ vs. sky /ɕyː/ vs. kino /çiː/no vs. tjue /çʉː/e vs. kjole /çuː/le and so on. Don't forget the short vowels. Get the pronunciation down by practicing and listening or by looking at a vowel chart https://en.wikipedia.org/wiki/Norwegian_phonology#/media/File:Urban_East_Norwegian_vowel_chart.svg and learning about places of articulation.
I think a typical notable problem that you might be having is distinguishing y, i, u, o. Plus, it is worth looking into the sounds that occur when r comes in contact with other sounds like n, l, t.