You need a proper clearing account with someone who can handle cross exchange. I'd recommend Sucden or someone like that if you are going to trade really small, or an Advantage, ADM, ED&F Man, GH Financial or similar if you are depositing USD 50-100k.
The clearer will then set you up as a child entity of theirs and can provide you with an ISV (which they charge for) CTS, TT, CQG and similar will be touted, all of which are ok for basic stuff (I'm on tbricks as we run algos). However, some exchanges have their own front ends. CME has CME Direct, and ICE has WebICE for example, eack of these costs in the region of USD 100 per month which does not break the bank!
Between the pair of them, you can trade oil, soft commodities, sterling, euribor, ftse futures, eurodollar and quite a few more.
The sales team at one of the exchanges will be able to help and point you in the right direction to be set up as a local. possibly a good place to start!
Do your maths beforehand though, while a CFD is leveraged, it is normally leveraged on only 1 contract. A contract on Brent Crude oil though, is 1000 barrels, so notionally is much larger than 1 CFD.
Henry.