Started going to the gym... gaining weight instead?

I have been going to the gym for a week now... doing 40-50 minutes of cardio along with eating healthy. So far I can't see a difference and when I stood on the scale today I gained a kilo! I don't have much weight to lose... about 2-3 kg but this is really putting me off now that I'm gaining instead. I really want to lose weight... what am i doing wrong?
Started going to the gym... gaining weight instead?
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