I recommend starting with short stories. "Space, Time and Nathaniel" and "Moment of Eclipse" are both great collections. Or just "Best SF Stories of Brian W Aldiss", which I think was updated several times -- that's how I first found him.
His novels are mostly excellent but I think there's a good chance you'll bounce off them if you don't happen to like the Aldiss vibe. He can appear very cold and dry until you realize how funny he (usually) is.
Second that... Non-Stop is one of my all time favourite SF novels. I'm not so sure that learning some of the details would spoil much - in fact the novel was in some edition published with a name that gave a lot away. But I'll avoid it anyway.
"Third" that. I loaned a copy to my wife with a hand drawn (ludicrous) cover that concealed the spoiler of the cover art. Ah, the Golden Age of SF, where the covers could be either completely irrelevant to the content or spoil plot twists... :-)
Two that don't get enough attention these days: "Billion Year Spree", his history of SF, and "Hell's Cartographers", six writers writing about how they write.
Avoid the Helliconia books, they're a grim, hard slog. Sorry I can't give a positive recommendation, I avoided his novels after trying to make it through those. Always enjoyed his short stories that appeared in the popular anthologies, though, and he's got some story collections that are probably good.
I thought they were brilliant and I greatly enjoyed them. Certainly they are not optimistic American-style SF but I wouldn't call them "grim" - they are grand novels about the cycle of civilisations, "deep time" and the impact of individuals (or not) upon those.
So did I. They are grim, but that's kind of the point -- it's a reflection on the tragedy of history and how people end up making the same mistakes over and over again.
When I said "grim, hard slog" I was referring to the experience of reading them, not the nature of their content. The experience of reading anything that monotonous would be pretty grim, even with a brighter theme and a happy ending. Of course, ymmv, etc.
I liked his novels. Intangibles was interesting, though not really SF. Also liked Neanderthal Planet. Few short stories where he played with the alternative history, like The Day of the Doomed King.
His novels are mostly excellent but I think there's a good chance you'll bounce off them if you don't happen to like the Aldiss vibe. He can appear very cold and dry until you realize how funny he (usually) is.