Luckily you can now buy bikini tops and bottoms singly, so you need something that detracts from your bigger bottom half. Don't buy tie-sided or skimpy bibkini bottoms. They'll just make you look larger. Get higher-waisted ones that fit properly. If you spill over the sides, it's not going to look too good. Then choose a top that 's pretty and eye catching. That way, you'll look more in proportion.
I agree, no stringy bikini bottoms, perhaps the ones that are more like shorts? I would definitely get a top that's more l ike a bra, to give you cleavage and uplift to make you appear bigger on top. Halter neck fastenings are good for making shoulders look broader too, again, to balance out the width at your hips.