(Taken from 2017/2018 CL Rules ) Page 23
https://www.uefa.com/MultimediaFile.../Regulations/02/46/71/38/2467138_DOWNLOAD.pdf
17.01
If two or more teams are equal on points on completion of the group matches, the following criteria are applied in the order given to determine their rankings:
a. higher number of points obtained in the group matches played among the teams in question;
b. superior goal difference from the group matches played among the teams in question;
c. higher number of goals scored in the group matches played among the teams in question;
d. higher number of goals scored away from home in the group matches played among the teams in question; e.
if, after having applied criteria a) to d), teams still have an equal ranking,
criteria a) to d) are reapplied exclusively to the matches between the teams in question to determine their final rankings.
If this procedure does not lead to a decision, criteria f) to l) apply;
f. superior goal difference in all group matches;
g. higher number of goals scored in all group matches;
h. higher number of away goals scored in all group matches;
i. higher number of wins in all group matches;
j. higher number of away wins in all group matches;
k. lower disciplinary points total based only on yellow and red cards received in all group matches (red card = 3 points, yellow card = 1 point, expulsion for two yellow cards in one match = 3 points);
l. higher club coefficient (see Annex D)