Hello aspirant!
you cannot join iSRO just after completing Bsc but you can join ISRO after completing your postgraduate degree as ISRO  demands B.Tech MSc degree. You can npt join ISRO just after b.sc . But you can go for M.SC , then you can go for ISRO. Or  , you can join as a technical assistant
. after completing MSC/PhD in physics you have high probability.
after MSC/PhD you can apply for Research scientist position and other fellow positions in ISRO.All you have to do is complete your bachelor and masters with high level of patience and dedication.Since ISRO is expecting a enthusiastic youth like you.
All the very best!