That's a fairly broad question so sorry for the shotgun advice.
You have to view your DNA matches and then look at your closest genetic cousins. You said you recognized a few? If so, if you look at their match page and click the "i " in the dark circle, it shows you how connected you are. Do you share enough DNA for that relationship? If so, you confirmed that grandparent or great-grandparent.
To confirm more, look at the matches you don't recognize immediately. Do any of them have names in their family trees that look familiar? If so, you can figure out how you're related and help confirm those shared ancestors.
For those you don't recognize, and who don't have family trees or they have trees that you don't recognize, it may take a bit more work to match them up with you.
You may have to work out your own family tree to include more distant cousins or you may have to give them some free research and build up their tree a bit until it connects with yours. Do any of them share your matches who you recognize? If so, you can figure out which part of the family tree they belong to.
Also, start with parts of their tree which geographically align with yours. If you both have ancestors in the same area, they might be related a generation or two back.
It also helps to include your own family tree on Ancestry and make sure you link your test to the test taker on the tree. This will help others find you.
The good news is that once you start identifying who cousins are among your genetic matches, future matches get easier since you can use the shared matches feature to figure out more quickly where they connect to you.