Friends and family is of course the main option, but for most younger entrepreneurs 20-25 years old our network may not be as valuable as somebody older.
Join accelerator. The primary value is not the educational stuff that they give you but the network they gather around their program. In our case, for example, one of the lectors was a serial founder and made an intro to VC that invested in us :)
#MatchingVoices Podcast - Season 1