First you go to your cam and check the maker / model Let us know which brand you have, we can't guess it for here.
Secondly you could check existing questions in the equipment section of your camera maker. Most people get the flash of the same brand to enjoy compatibility and full features. Another option could be the price guide, 3rd party equipment is listed there as well. Last but not least, have a look at the Mass Sales section and virtual shops in Marketplace. Plenty of original brand and 3rd party offerings.
For reviews: just key the flash model into Google, combined with the magic word 'review'. Happy reading
Some of the factors are:
1) Price (obviously)
2) Output power (usually the "Guide Number" is used, but make sure the values for distance and ISO are the same to have good comparison)
3) TTL capability?: Flash output can be controlled automatically by camera body
5) Ability to swivel in all sorts of odd angles
and so on...