Fill the tire with air, if it won't fill then you'll have to get the tire mounted. If it is just flat simply add air.
Then with a small spray bottle (old windex bottle or such) put a tablespoon of dish soap and fill half way with water. Swish it around. Spray the filled tire with the bottle. The tire will bubble where it is leaking. As stated above if the leak is on the side wall it's not recommended to fix.
If the leak is in the tread you can simply get a plug kit. Very very easy to do yourself. This kit it approx $8 and will take about 4 minutes to fix. After you plug it, then you can add some green slime as an extra precaution.
If you have a rim leak it could simply have dirt on the wheel preventing the seal you need. At this point you should have the dealer remount the rim to see if they can take care of it by cleaning the rim and resealing. If the rim is bent, well looks like a rim is in the future.
Also you can get a tube to put inside your tire. A little more work involved there, but it's doable as a DIY project.
It's all pretty easy just ask guys on here or YouTube videos.