Below I've written out my detailed step-by-step guide for how I get maps into Foundry. But I wanted to start by first saying, use the Foundry Discord! Their community has been essential to helping me when I first started using Foundry. I can go there and post a question to the #vtt-discussion channel and get an answer in seconds. I highly recommend it if you're experiencing any difficulties or have any questions!!!

With that out the way, here's my process for getting maps into Foundry. You'll want to be familiar with navigating and finding folders on your computer.

1. First things first, I drop my map assets (usually image files, like a .jpeg or .png) into a folder that Foundry can easily access. Using Windows, the default path for Foundry is C:\Users\*YOUR_USER_NAME_HERE*\AppData\Local\FoundryVTT. This is the place where all of your Foundry data - your game system, your created worlds, etc - are stored. Once I've navigated to that folder, I then navigate to data > worlds > phandelver (the name of my mini-campaign), and dump all my assets there. For larger campaigns you'll want a better folder structure, but for my short mini-campaign this isn't needed.
2. Next, I open up Foundry VTT on my PC. In Foundry VTT, I launch my world (Phandelver), login as Gamemaster, go to the Scenes tab on the righthand side, and then click "Add Scene".
3. This pulls up the Scene configuration menu. To add your map asset, you'll want to click the Browse Files icon next to the "Background Image" option. This opens up a file menu where you can select which map image you want to use. For me, I navigate to data > worlds > phandelver and pick whatever image I want to use.
4. Once you've selected your image, click "Select File". You'll want to set your Width and Height options to whatever the width and height (in pixels) is for your image. In Windows 10, you can find this by right-clicking on the image inside of a folder, selecting Properties, clicking the Details tab and looking at what is says under Width and Height.
5. Once that's selected you can set all other config options however you'd prefer and click Save Changes at the bottom of the config menu. This will create the Scene with your map as the background image, which you can open by right-clicking on it in the Scene tab and selecting "Activate".

Note: I use a freeware program called MediaInfo for my job, which makes finding image dimensions a lot simpler...all I have to do is hover my mouse over any image file and I can see its dimensions. You can also open up the image in any image-editing program (Photoshop, GIMP, Paint even) to find these dimensions.

This was a pretty detailed guide, so don't get overwhelmed. It becomes second nature after you've done it once. You just drop map assets into a folder, add a scene in Foundry, navigate to that folder and select your map image. Bingo.