Je vais très bien: I am very good
Je vais tellement bien! (no old person would approve of this as being correct French - in this context, "tellement" is used by youngsters to emphasize how cool something is.): I am so good!
Generally and in normal correct French tellement is just the equivalent of so.
It's so beautiful. C'est tellement beau.
It's very beautiful. C'est très beau.
Tellement > très
Another one: "c'est tellement cool!" again, used by youngsters: that's so cool
In this case it's like saying "c'est trop cool" which would translate to it's too cool.