The main thing with testing is to not get your hopes up and have patience. There are understandably a lot more volunteers than products available!
Top tips can be found in the The Great Big Review and Testing Guide 
I’ve personally been lucky and had quite a lot of testers over the years, having said that, I’ve been around a pretty long time and have bought and reviewed a variety of different products
I’d recommend only putting your name forward for things similar to what you’ve already reviewed rather than something totally random, and keep reviewing and interacting on here 