1. Plan ahead.
If you don’t have any higher education language qualification or specific translation qualification, I believe this is a must before you get started. This may mean some study time before you can get going. (Click here for my previous post for info on the Institute of Linguists Diploma in Translation.)
2. Get experience.
Offer your nascent skills to charities to get translation experience. Some have regular need for translators, others just need things translated on an ad hoc basis. I worked for a charity called Latin Link, where I did a number of translations and was using my language skills on a regular basis. Write to charities you know have activity in the countries where your translating language is spoken, besides the experience, you will find giving rewarding!
Join professional associations like ITI or IoL, even as just an associate member and translation communities like Proz or Translators Café as a non-paying member, then attend meetings, powwows, conferences. No need to go to everything going, but just somewhere where you will meet translators who you can ask about their work and get advice.
4. Get good equipment.
Make sure your hardware is up to scratch. Slow computers slow you down, simple, as does a slow internet connection. Both of these are not expensive to resolve. I recommend a laptop over a desktop. As a freelancer, the ability to hotdesk is a real benefit. The other thing I love having is a second screen. Your translation can be on one and your source document on the other.
5. Get good dictionaries and resources.
Of course there is a lot available for free online, but specialist dictionaries are not. If you know what area you are likely to be translating, invest. Some dictionaries come with a CD-ROM version, or allow you to download a digital copy from their website, this is REALLY helpful. My Collins Spanish-English dictionary gave me the latter.