Yes, u can eat right after putting ur braces... tho u might be feeling a bit uncomfortable bcoz u will feel ur mouth is full of brackets... u can try to eat soft foods... tofu soup, mashed potato etc... or drink something that can full ur stomach... for few weeks u might lose ur weight due to soft food diet... but i believe theres lotsa "unhealthy" yet soft food out there for u to gain ur weight e.g. choc, full cream milk, soft drinks, ice cream, rice, pasta etc... i still remember my trainer at gym said that, for underweight ppl, just eat everything, n dont control... so its bonus point for u to eat things which many ppl avoid in order to maintain or gain ur weight... just make sure to clean and brush ur teeth after that...

However, as soon as u feel comfortable with ur braces, u can eat almost everything... burger, pizza, fried chicken etc... its hard to bite directly as ur bracket might loose or wire broke, but u can cut it into small chunk n chew slowly...

Good luck