To Be Finished, To Be Completed
intransitive verb, godan verb
You've seen the word 出来る (to be able to do), which is the first part. Then the second part, 上がる means "to rise." So, think about it this way: once you're able to do something, you rise because you're finished. That's why this verb means to be finished.
できあがる
The reading is a combination of the readings from 出来る and 上がる. So long as you were paying attention in lessons past, you should be able to read this word as well just by combining those other words.
工作が出来上がるのに、三日かかりました。
It took three days for the crafts to be completed.
お茶は大体五分で出来上がります。
Tea will be ready in about five minutes.
家は月末に出来上がります。
The house will be completed at the end of the month.