1. To become heptathlon champion at the Sydney Olympics was a dream come true.
The entire underlined portion is the subject of "was"; "a dream come true" is the subject complement (i.e. it tells us more about the subject).
It is quite common to use a to-infinitive clause as the subject; cf. these examples (all from Google):
2. To teach is to learn twice.
3. To know her is to love her.
4. To weep is to risk appearing sentimental.
When you see a to-infinitive clause as the subject, it means the action expressed by the infinitive is the subject.
In your sentence, a gerund would also be fine (perhaps better):
5. Becoming heptathlon champion at the Sydney Olympics was a dream come true.
Does that make it any clearer? Let me know if not!
All the best,