what's healthy to munch on?

Gemma
Hiya so I'm 16 .. and I'm 7 months into a 2/3 year apprenticeship for an Engineering consultancy. (Studying civil engineering).
So I have put on a lot of weight recently.. we're even my parents are starting notice. I'm not fitting in any of my clothes and I feel now uncomfortable. Just signed up to the gym but the main problem is I sit at a desk all day and am munching something.. anyone got any ideas on what I can munch throughout the day that is healthy for me???