I have begun reading more manga about relationships, but I don't have a clue about how to begin looking for a manga that may be like what I want to read about.

Basically, I am looking for a manga about romantic relationships between adults where the guy has never fallen in love before. I don't know if it would be stretching things to ask that the guy not be some kind of asshole, but it will be what it will be. I'm not too sure on any other particulars, since I am fairly new to romance as a genre in manga.

Thank you in advanced for your recommendations.

bonnouji, if you want/don't mind something light-hearted.

Try Futari no Renai Shoka.

Well ain't gonna get much better than Bonnouji
If you're new to romance you could take a look at my lists, apart from the really recent stuff it's all rated. About 95% is romance so should be hard to miss them.

Taiyou no Ie would fit I guess, but it's from the girl's perspective (mostly)
Watashitachi no Shiawase na Jikan is a classic (on here at least)
Itoshi no Karin I think he was never in a relationship

in need of romance?
uncommon, sad, sweet