If you are already in the UK as a Tier 4 (General) student, your dependants can apply from their home country to join you in the UK. However, if you are in the same country as your family members, you must make your visa applications at the same time.
If your family members are outside of the UK, the application form can be found at the UKVI website.
Just like in your own Tier 4 student visa application, there are maintenance requirements that each dependant must meet.
For further details about making a dependant visa application and the requirements, please refer to the UKVI Policy Guidance for Dependants. This should be reviewed carefully before making the application/s.
The UK Council for International Student Affairs (UKCISA) also has a useful guide for dependants available on their website.