While waiting for someone who's more familiar with Voltaire, Nicholas Cronk's interview provides some context to Voltaire's career and gives some book recommendations. The two primary works he recommends are Candide and A Pocket Philosophical Dictionary. He explains his reasoning for both pretty well.

Fwiw, if you're just looking to start reading Voltaire, I don't think you'd go wrong reading the Candide. It's an enjoyable, short, and funny satire. The only context you may want first is Voltaire is largely responding to a view around in his time commonly called 'the best of all possible worlds'. This article explains that a bit better

If you're looking specifically for his philosophy, the SEP contextualizes what Voltaire has written and where. Its bibliography has primary sources, so you could always start with the one that you find most interesting