First off, I am by no measures an expert on this. I've been doing this for 2 months and have sold 12 things on Amazon. I don't know if this is good or not. But I am happy to share my process with you.
Think about what type of thing you want to write about - I'm a stay at home mum so I feel comfortable with baby and kitchen products. Take a look on Amazon.com and have a browse. Looking at the bestsellers is a good idea as I think these will convert well. Pick a type of product, for example, dehumidifier.
Go to the Google keyword search tool. You can find it by Googling 'keyword search tool free'. If you haven't used this before then there are loads of hubs written about it that will help you. Do a search on hubpages for hubs on 'keyword search tool'.
Use the tool to find keyphrases linked to the product you chose. You are looking for a phrase that has a good number of searches in the US every month, but when you check out the Google results for this search term, they are not too high. Again, there are quite a few hubs about this. There are differences of opinion as to what sort of figures you should be looking for, and since none of my hubs have hit the top page on Google yet, I am in no position to make recommendations.
You may be able to group a few keyphrases together to make one hub, for example you could do a hub on dehumidifiers and include the keyword phrases 'Delonghi dehumidifier', 'Black and decker dehumidifer', etc etc.
And sometimes, like with the strawberry huller, I just stumble across something that is cute and do a hub about it and worry about keyword phrases later.
Anyway, that's how I got started. I've still go tonnes to learn. Read hubs by Nelle Hoxie and RyanKet for starters. Nelle is the product hub legend we all aspire to be.