Weight+muscle+other stuff

I'm gaining fat at my recent job at MacDonalds, but gaining muscle mainly in my leg and arm region, Is this bad or good, or both? I get 50% off MacDonalds, and now I'm cutting down on what I eat there (got a free meal today) I'm not sure if this is ok, I do 20-30 push ups, 20-30 sit ups and 1-2 minute planks (google this if you don't know, it's to work on core strength) is this fine or do I need to do much less or more? I do not like working outside, as I rather stay home, I'm only 16 (in may) I really don't care about my look, I actually just want to get a bit stronger.

I enjoy doing push ups, testing at school I did 31 push ups in 30 seconds, Running.
I don't weigh myself, I'm acutally extremely happy, I have heaps of friends, I'm classed as a "Cool" group, but I'm sota chumby and also ugly (don't care, I will get all the girls, I have girls that like me so I must be a beautiful inside)

I just want improve my strength, just in case my brother or friend gets in trouble
(impress the girls)