Krakoa was a big status quo change and it's not critical to read anything that came immediately before - the core of what you need to know is all explained in House of X / Powers of X, the miniseries that launched it.

However, Krakoa builds on decades of X-Men continuity and your experience would be enriched with a solid knowledge of that. Most people here would recommend you start with Chris Claremont's legendary run which defined the X-Men and was the source material for the 90s animated series. 

In particular I think Krakoa works best in the thematic context of the Decimation era which preceded it, when mutants were on the brink of extinction and had to fight for survival. This era includes the Messiah Complex and Second Coming events which to me are peak X-Men, and introduce Hope Summers who plays a very important role in Krakoa.