There isn't really any set of songs that are considered easy for everyone. Here are some things I know, though:
- 40mP, koyori, JimmyThumbP, Honeyworks, MikitoP, Souta, and keeno songs have a reasonable range for the most part and are slightly repetitive, which makes them easier to learn.
- Kagerou Project and Neru songs are extremely challenging; the ranges are wide and the melodies are slightly unpredictable. Not for beginning singers if you want to produce quality work. Though you will probably feel awesome if you sing them! In fact, most famous "series songs" (Evillious, Shuuen, PanVoxx) are what I would consider "intermediate difficulty".
- Don't think of this as lowering the bar; it's more like allowing you to redirect your energy to getting used to the process and working on the quality of your singing.
- If you plan on singing in Japanese and it's not your native language, you have to work to get good at singing in Japanese. Practice practice practice! Listen to other covers and your own recordings and compare the diction, especially vowels, which differ ever so subtly from English vowels. Don't try fast songs until you're ready. It might be really difficult, but try your best to memorize the song you're singing, since it make you feel a lot more confident and frees up your brain to think about other singing-related thing.
- Make sure that the song is within your range. It's fine to have a few notes sitting on the edge of your comfort zone - that's how you grow as a singer - but baby steps line the path to perfection.
- Sing a song that you like! That way you have the motivation to sing it to death and listen to yourself a thousand times and therefore improve.
- Experiment! Try styles that suit your voice! There are awesome songs that are J-Pop, anime style, horror, rock, Japanese traditional, R&B, jazz, ballad, and everything else on NND. If you have any special traits that you're fond of, find a genre or a few that will let you bring it out!