Would an h100i fit in my case?

Nope, h100i needs 2 x 120 mm fan mounts right next to each other. A h80i only requires 1 x 120 mm, so that might serve you.

Thanks, but I also need to know if I could fit a hyper 212 evo

And also, somebody told me that I could fit some computer fans on the side vent. Is this true? And would this work for the h100i?

No - Hyper EVO 212

No - Side vent.

Your case is too small.

Oh, not the side vent. The case panel opposite of the mother board. There are all those holes and he said I could just do something like use a piece of string to hold it in place. Would that work? And also, the hyper 212 evo should be able to fit in my case. I measured how much space there is between my CPU and the top and it is 160 mm which is the size of the heat sink.

That sounds very risky and very tight, you could try, but I wouldn't recommend it.

Which one sounds risky? The fans on the side panel or the heat sink?