Questions & Answers about Solo quiero una rebanada de pan con queso.
Spanish often leaves out subject pronouns when they are not needed.
Quiero already tells you the subject is I, because it is the first person singular form of querer.
So:
- Quiero una rebanada... = I want a slice...
- Yo quiero una rebanada... = also correct, but yo adds emphasis, contrast, or clarity
You would use yo if you want to stress I, for example:
- Yo quiero una rebanada, no él.