argan oil then use a straightener
If it's just a short term straighten then use Pro naturals moroccan argan oil or whichever brand of argan oil you prefer. I use pro naturals because it's light and is also a heat protectant.
Then use a straightener and the oil should really help your hair to smooth down and for the straighten to last longer. I use a Karmin G3 because it reaches 460°F and my hair is terribly difficult to straighten.
If you want a longterm straighten then check out Cocoa Keratin Treatment. It's very safe for hair and it lasts for about 5 months. It comes with the whole treatment system (shampoo+conditioner) so you'll have the best care.