Taking photos of the night sky

Recently I have seen some really good photos of other galaxies and so I thought: WOW! There was a description: iso of about 800, 4-5 minutes and some photos were taken with less than 200 mm. So I thought: hmm... that can't be that hard. So I tried it out and realized that it is very difficult. When I shot at 300mm I realized I didn't have the basic accessories or knowledge to make them. So I put on 18mm and left it for about 30 minutes. I also used semi-fish eye.