How may miles/kms are on your vehicle Kris? A slipping automatic transmission is a symptom of either low fluid or just plain ol' tired AT fluid. First, put it on level ground, idle in park until the engine is warm, then find the AT dipstick (near the rear of the motor on a RWD or nearer the driver side of a FWD vehicle. It should look somewhat like to the oil dipstick. Is it at the proper level. Is it clear & pinkish (newer) or dark & stinky like burnt (old and tired)?
If it's just low yet still pinkish... top it off WITH THE CORRECT ATF.
If it's dark and stinky... time for an ATF "flush"... check with your mechanic. With automatic transmissions, you can just drain and add fluid but that only changes about 2/3 of the fluid is trapped in a torque converter. If you use this method you have to do a drain/fill procedure with about a 10km drive in between to get the old/new ATF to mix and essentially better and better. This might cost you $80-$120 or so. Alternative: Find a nice neighbourhood mechanic. To ensure that all the fluid is "exchanged" with new ATF, some aluminum lines that go to the radiator cooling tank have to be tapped into and while the motor is running, new fluid is sucked in one pipe and the old stinky stuff comes out the other until its is the fluid runs clean. This would cost $150-$250ish. (I DON'T recommend a forced flush with cleaning solutions as this may loosen some chunks, bits and pieces that then get jammed in valves and small passageways). . Then... if you/re rich, you could take it to a dealer.
MY SUGGESTION: DON"T immediately take it to a dealer / oil change place right way. They often take of the cover plate then show you the dreage/gunk on the bottom of the ATF pan and maybe even show you some slivers of metal on a magnet that is on the plate then tell you... you need a new transmission. (Often these metal bits are remnanants from manufacturing/drilling OR some wear on the gears. Do a single drain and fill. Is the tranny still slipping ???
A fairly good site "Dummies" site that has some good instructions.