There is not a thing as "best tactic" what works with one team might not work with another and what works against one opponent may not work against the next.
That's why you need to do a lot of experimenting, trial and error, usually my whole first season with a new team is me experimenting with different formations, team instructions and individual instructions. And also you probably going to need more than one tactic maybe one for easier at home games, one for difficult away games against stronger opponents and one crazy all out attacking one for then you are a goal behind with 10 minutes left to play.
If you google FM18 tactics you will probably find a lot of tactics that other people have created, if you are lucky maybe one of them works with your team, but to be successful in the long run don't be afraid to experiment and study all the information you get during the games.
Try to find out what is going wrong, here is two examples:
1. If your players missing a lot of the shots, maybe it's because they are shooting from a far away distance while beaning really bad a long shots. Try change the team settings to work the ball into the box and at the same time putting individual instructions to the worst finishers to shoot less. Sometimes this may fix the problem, other times it may not.
2. If your opponent is focusing all their attack on the flanks and nothing at all in the middle while you are using a defensive midfielder, maybe he is not involved at all and you can putt your full backs to focus on defense while bringing your defensive midfielder higher up in the pitch helping out with your attack without weakening your defense.
If you need some more tactics help maybe this forum thread could help you:
Football Manager 2018 Tactics Guide