How to supe up a Ford Fusion style 2009?

I've just purchased a Ford Fusion and would like to increase bhp, pretty much make it a sleeper. How would one go about it?

You mean "soup up"? That is what we call "hot rodding" It goes way back to the mid 20th century. The original hot rodding method was to remove anything possible not absolutely essential to the operation of the car: hood, fenders, back seat, etc. That was to get rid of weight. The next step was to free up the intake and exhaust. Remove the muffler, put a bigger carburetor on it. Most people stopped there but serious hot rodders swapped out the engine - the usual was a Ford, flathead V-8 and they often did internal work on it such as milling the head for more compression.

Modern techniques differ quite a bit but the basics are still there. But I urge you make sure that the basic systems of the car that are essential for safety are in tip-top condition before you add power to it. By that I mean the brakes, suspension and steering. Add another 20 horsepower and go tearing down the road only to have a worn ball joint or tierod end come apart and you and your car could end up in the boneyard.

Join a Ford Fusion forum. I'm sure that there are several. There will be all kinds of people there who own Fusions. There will certainly be a performance section where you can learn all kinds of tricks and discuss issues with other knowledgeable members.

Don't bother. Trade it in for a better car. Adding power without matching upgrades to brakes, tires and suspension will an insurance issue.

Hi so tuning up a car is never going top make it a sleeper.

First of all, I hope it is a stick!
The 2.3 engine is a "stroked 2.0", depending on what you can find locally, swap the engines or stroke the one you have. If possible find an "eco-boost" engine and ECM.
Up-grade to a light weight fly-wheel, stage II pressure plate, but stick with a full clutch face. Do not go with a puck style disk, they burn out too fast.
Do some shopping for a wheel and tire combination that is as light as possible. Some impressive looking wheels can weigh ten or more pounds over what a less impressive wheel does.
If you have big bucks- then consider a V-6 engine to drop in. There are too ways to go faster, one is cubic inches and the other is big bucks. While the Eco-boost engine of the Mazda L engine is fast, a V6 engine is more durable and in the long run cheaper if the swap is with in budget.
Ditch the sound system or at least don't put a better one in.
Sometimes Grassroots Motor Sports does a project car. They might have done one. But keep in mind they want to promote the products of the advertisers. So sometimes they actually put parts in that slow the car down or do no good. For example the air intake stock is a cold air intake. So you don't need one. But a intake scoop below the front bumper (passive forced air induction) does help and parts from Lowes for "wet vac's" work just fine.
For suspension consider going with Gabriel Shocks. A bit difficult to find, but the ones that are one grade up from "stock" are sold by another name for a lot more as performance shocks.
A small lip and mild splitter would not hurt. But the thing is to actually get the benefit from them you would have to violate the basic speed law.
Keep in mind Ford and Mazda have tweaked that engine about as far as can be done! There are no quick routes to faster performance and it is getting more difficult to get the edge with out spending some money and time working out details (the V6 swap is not easy, but possible).

By buying a Mustang

Sell it and get a Mustang.