Sure fam.
Whatever rank you received at the end of the regular season will be your rank for season 6.
During preseason, you can continue to practice ranked at the MMR you were at at the end of the season. Preseason games will increase your MMR, just not as drastically as regular season games.
When season 7 starts, Riot will soft reset MMR, which basically means that everyone will be pushed towards Silver4, as it is the center of skill bell curve, or 'average'. After the soft reset, you can play your placement games. Placement games start you at your soft-reset MMR and rapidly adjust as it quickly attempts to find a good starting point for you for the new season.