I am new to NBA and I would like to know if teams in Western Conference are globally better than those in Eastern Conference or if it is the opposite ?
There are a couple ways to look at this, but since 1998, the Western Conference seems to be overall better than the Eastern. If you look at this site, you can see that in the 18 years from 1998-99 to 2015-16, the Western Conference has collectively had a better record vs the other conference 16 times; that means the East only bested the West twice in that span. Even in those two years, the East margin of victory was very small.
If we look at which conference won championships, the margin is a little closer but the West still comes out on top. From 1998-99 to 2016-17, the West has won 12 titles, and the East has won 7. This is much closer to 50-50 like I said earlier, showing that the disparity between the best teams in each conference is less. However, the West still seems to be the better conference.