My best sdvice is to buy the best kit you can afford. Similar to fiber related stuff, cheap doesn't always turn out the nicest. If you are US based, I highly recommend the canvases from Diamond Art Club (DiamondArtClub.com)--they are more expensive but also are amazing.

There's a lot of people you can watch on Youtube who do unboxings and reviews of canvases. I really enjoy watching Rambling Rose and Diamond Painting (that's her channel name), Mrs Crochet and Coffee, Rachel Rae, and WhitneyDiamondPaints. Rambling Rose and Diamond Painting gives really great reviews.

If you are looking on Amazon (or just on the web in general), Tinmi Arts, EverydayEDeals, and Fansells have good canvases.

Most importantly, pick a picture you really love!