How to be an aggressive defender in Soccer/football?

I've been a defender for a few years now. Last year I was on my school's JV and V now this year I am full Varsity. I am playing right back instead of the middle like I always played so I'm not use to picking up somebody on the side and trying to contain them as much. I want to know how I can keep them on the outside and how aggressive I can get with them (before I can called or carded)
 
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