Your requirements are very vague, and there are a multitude of options. Which is the best is very much "it depends". Options include:
- Transactional Replication. (To get all changes.)
- Change Data Capture. (To get all changes.)
- Change Tracking. (If you only want to track that something has changed, but not get all changes.)
- Having a trigger that writes to the target table (Probably not a good idea if the table is written frequently.)
- Trawling for changes by using a rowversion column or a LastUpdated column. (Akin to Change Tracking, but more prone to miss changes.)
- I've probably forgotten something.
Beside the impact on the system, one thing is also to consider the skillset in the organisation. Change Data Capture could very well be the best choice technically, but if your org has never used CDC before, there is a hurdle.)