Mixing "hot" colours is often difficult. If you add white to most colours they'll merely become paler, while if you add black or other darker colours, your result will likely be a deeper but murkier colour than what you had in mind. You can go to an art supply or hobby store and buy fluorescent paint; stores that carry this kind of material are likely to have hot pink or something like it in stock. However, if you're intent on mixing hot pink, or a close approximation, you can use colours you may already have in your paint kit.
Lay out some alizarin crimson on your palette. You can find this colour in oil paint, acrylic, tempera, gouache and watercolour. Regardless of your medium, the result will be approximately the same.
Add a small amount---about a match-head's size---of titanium or flake white to the alizarin crimson. With oil or acrylic, use a palette knife with a flexible blade to add white and mix the two colours. For gouache and watercolour, use your brush to move and mix colours. Rinse the brush before you dip it into a new colour.
Mix in small increments of white until the paint begins to approximate the shade of hot pink you seek. If you're trying to match the colour to something, coat the palette knife blade or brush hairs with the colour you're mixing and hold it near the surface you're trying to match.
Add small amounts---half a match-head sized---of cadmium red if you find the mixture you have is too anaemic. To make the colour a bit deeper, add a wisp of pthalo blue, but make sure you use this colour sparingly. It's extremely dark---darker than black---so it won't take much to change the character of the colour you're mixing.