Just a couple of quick tips, as a complete tutorial on this would take several pages and the general concepts are widely available elsewhere on the net - just do a little Googling.
1. .mp3 is the most widely used format and is suitable to most people's needs, so yes, from that standpoint it is "the best".
2. How much space it takes depends on what quality (as expressed in kbps) of encoding you do when you convert your CDs to .mp3s. 128 kbps, 192 kbps, 256 kbps and 320 kbps are the most common settings. The higher the encoding value, the better the theoretical sound quality, the bigger the file size and the more space it takes. I have used primarily 192 kbps on my collection and at that setting, you can store approximately 400 CDs on a 32GB Micro SD card.
3. The easiest way to get them on your device is to encode the CD to .mp3 format, store them on your computer, then connect the Infuse to your computer via USB, and copy and paste them from your computer to your Infuse.
If all of this is new to you, you will need to do some online research in order to know how to convert to .mp3 and copy and paste to your device.
Best wishes.