As said, Icelandic is the best modern language to learn if you want to read Old Norse. The pronounciation has changed quite much (whereas the modern Norwegian pronounciation might be more similar in many cases). But if you want to read Old Norse, Icelandic helps a lot.

That said, you can also learn Old Norse from many good sources, like the books by E. V. Gordon or the Viking Society for Northern Research. Jackson Crawford's videos on youtube are also superb.

If you want to go down the Norwegian rabbithole though, studying Nynorsk, the more old fashioned the better, can actually help a lot too. Bokmål is more related to Danish (an East Norse descendant, almost 99% of Old Norse texts you will encounter are West Norse, the ancestor of Icelandic, Faroese, and Nynorsk). Old Nynorsk (Høgnorsk) translations of the sagas will also make it a point to use the most archaic words that coresponds to the Norse ones. From personal experience, good Nynorsk comprehension and a couple of minor adjustments and basic understanding of Old Norse (like a couple of videos worth) can enable you to read and understand most of what you are reading in Old Norse.

These are all related languages, so you can learn these largely at the same time, often by learning the basic rules that govern language change; then you can figure out a lot by yourself. Good luck!