me too, I'm learning dutch , but u could try listening to music or the weather forecast or the news or maybe some kind of a nice dutch films or series if u hear a lot . ... that should do it try copying the things u hear or try speaking with a native dutch person and ask if he or she could correct you if u make a mistake good luck !
's+j' is a difficult combination for us, too. That's why in this case we usually pronounce the 's' like the English 'sh' (the 's' assimilates with the 'j'). You can hear the difference between 's' and 'sh+j' in these examples:
fles - flesje (bottle - little bottle)
roos - roosje (rose - little rose)
wat lees ik? - wat lees je? (what do I read? - wat do you read?)
The same goes for the combination 't + j'. In the combination 't + j' we pronounce the 't' as English 'tsh'. You can hear the difference between 't' and 'tsh + j' in the examples underneath (remember final 'd' and 'dt' are pronounced as 't' in Dutch).
We pronounce the 'r' in a lot of ways, one of them being a rolling 'r'. Not many Dutch people can roll the 'r' by making a thrilling sound with the tip of their tongue. So we are always a little jealous of the people that can. If you have a Spanish 'r', you could try to make it a little less prominent than you would when speaking Spanish. I have no idea if you can say a word like 'groot' with a rolling 'r', though. :) I have three different r's that I use depending on the place the 'r' has in the word. I will post examples later today. I am from the West of the country (Amsterdam region). In other parts of the country people pronounce these r's differently.