What an ISRC actually is
An ISRC is a 12-character code (like IN-XY7-26-00001) attached to one recording. It follows that recording everywhere, so if you change distributor later, keeping the same ISRC means your stream counts and playlist adds carry over. A different mix or a remaster gets its own ISRC; the same master keeps one for life.
The easy way: let your distributor assign it (free)
For almost every independent artist this is the answer. When you upload a track, your distributor issues an ISRC at no extra cost. You do not need to apply anywhere. Check your release dashboard, the ISRC appears there, and note it down for your records.
The direct way: become your own ISRC manager
If you release at scale or want to control your own codes, you can register as an ISRC manager and be issued a registrant prefix, then generate your own ISRCs for every recording. In India this goes through IFPI's appointed national agency. This makes sense for labels and prolific artists, not for a first single.
Whatever route you take, keep a simple spreadsheet of every track and its ISRC. It is the backbone of getting paid correctly.
Do not pay a reseller
Some sites sell ISRCs individually at inflated prices. You rarely need this: your distributor's free codes are valid and permanent. Paying per code only makes sense in specific label workflows.
ISRC is for your recording. It is separate from your composition rights and IPRS registration, which cover the song itself. You need both handled.