Hi David, covering a hole in your roof with a tarp will not be a lasting fix to your situation. Here's how to patch a hole in a shingled roof: http://thehtrc.com/2017/diy-roof-patch
What type of roof? Composite shingle? Clay tiles? A tarp is a temporary fix at best. Will not be weather proof and cause more harm than good. You need a proper licensed roofer. If not repaired properly, you run the risk of water intrusion and resulting mold which is far more costly to remediate than paying for a roofer to do the job right the first time around. Here is how to hire any contractor you will ever need.
For a temporary fix to keep out rain until the roofing contractor arrives get a rip stop tarp large enough to both cover the hole and extend over the roof ridge. Put the tarp over the hole and up over the ridge and secure in place using cement blocks or heavy long boards. You want to make sure that everything "uphill" is sealed so water doesn't run under the tarp.
Agreed- get it fixed for real or you're going to have much more serious problems soon.