To stop water leaking into my plastic shed.

Depending on what type of roof it is will answer your question more precise.

Depends on roof type your talking and where ingress of water is coming from, photos are a godsend

if the hole is small then some waterproof sealant adhesives are available for this job.

There is various options available you can either seal with a non acrylic sealant put a fiberglass repair patch on with resin bond or use a clear seal patch similar to those on hot tubs ponds ect