It all depends on what you want to ride.
If you're wanting to go straight into trials and drop all of your BMX riding, Then get a 20" (You'd probably find a short one better, if you see one you like just make a topic and everyone here will advise you from there)
If you're wanting to keep the BMX style of riding then go for a 24" street bike.
I wouldn't recommend you even look a 26" bikes XD
As far as "what to buy"
I started on an onza rip, which I regret given I never had much money to buy a new bike when it was no longer suitable for me, meaning I was stuck with it.
I'd advise you to go second hand, I buy almost everything second hand, you get way more for your money!
Don't be attracted by the low prices of 20" Onza bikes.. often they tend to be poop unless you upgrade the crap out of them.
so yeah, Take a look on here, for something, or Pinkbike. Ebay and gumtree are ok but bike thieves tend to go to them so I normally stay away (I have bought bikes off them in the past though, and you can find genuine sales which are a bargain so take a look anyway).
Hope I've helped.