what is the rule for ties in the CL group stage?

[Sxrenity]

Goal difference first - if it's equal then it goes to head to head record.

[rastaman70]

here is the real info to determine who advances in tie breakers from UEFA:

Tiebreakers
The teams are ranked according to points (3 points for a win, 1 point for a tie, 0 points for a loss). If two or more teams are equal on points on completion of the group matches, the following criteria are applied to determine the rankings:[1]

1) higher number of points obtained in the group matches played among the teams in question;
2) superior goal difference from the group matches played among the teams in question;
3) higher number of goals scored in the group matches played among the teams in question;
4) higher number of goals scored away from home in the group matches played among the teams in question;

If, after applying criteria 1 to 4 to several teams, two teams still have an equal ranking, criteria 1 to 4 are reapplied exclusively to the matches between the two teams in question to determine their final rankings. If this procedure does not lead to a decision, criteria 6 to 8 apply;
superior goal difference from all group matches played;
higher number of goals scored from all group matches played;
higher number of coefficient points accumulated by the club in question, as well as its association, over the previous five seasons.