You have to be really careful when looking to become an SEO writer. There are numerous places that are looking to hire but if you aren't careful you can often end up working for a penny or less than a penny a word. I used to work as a freelance SEO writer, I was doing anywhere from 200-400 articles a week ( sounds like a lot but Dragon speak is a miracle program, at 120 words per minute its not hard to write that many articles in a week). This was what was necessary to make a minimal income that barely managed to pay the bills. One of the biggest problems you face when getting started is competing with countries that can bid on large projects for sometimes as low as $0.20 per 500 word article. This has been my experience and I used to work with about 200 writers at my last contract company, all of them had similar experiences.
In regards to the requirements of the articles, that depends on the client. Some of them will provide you with a list of topics and you will be responsible for finding appropriate keywords, some will contain a list of keywords, and in some cases you may find yourself writin 10-20 articles at 500-1000 words a piece on exactly the same topic and keyword while attempting to make them all unique.
It can be challenging and I found it rewarding in many respects, though not always financially. If you are looking into this seriously, consider contracting with a company, there are plenty of them out there, primaily to help protect yourself. Many people use bid sites, forum sites that are designed to promote writing jobs, even Craigslist and if you aren't careful you may end up doing and submitting it only to find the client has disappeared overnight.
Working with a larger company offers a few benefits, the first is that you don't have to worry about getting paid, many of these companies have a monthly subscription fee for clients and also charge per article. They also help protect you against fraud and other legality issues, which can sometimes arise when doing this type of work. Finally, they can take out the time consuming business of tracking down clients and marketing leaving you ( the writer) with more time to focus on writing instead of dealing with the additional hassles of marketing, accounting, and customer service.
Hope this helps.