I lie to my boyfriend and now he said that he don't trust me anymore. what should I do to get his trust back??
Depending on the severity of the lie(s) in his personal opinion and also how giving he is of his own trust. It could be as simple as an apology or a long period of heartfelt regret and gentle, but loving, persuasion.
If you are willing to share more then it could help with a more specific answer. But i would start foremost with a sincere apology and then ask if the two of you can sit down and talk it out.
with time and your action's. this is someting you crumbled down and now only time can build it back up.