1. workshops. preferably through AAFP (American Academy of Forensic Psychology). this will also allow you the opportunity to network. I've generally found forensic folks are the most laid back and down to earth (even the big wigs), as well as open in providing consultation (of course, I'm biased a bit).
2. If you're not already a member of your state psych association, join. this will allow you the potential to meet more folks actually doing the work in the field, and also potentially find out if any might offer supervision services. Generally speaking, it would be best to get supervised experiences on actual cases before diving right in.
3. get on the psy-law listserv
4. join APLS through APA.
5. Read Melton as a start.
6. dont get fooled by any fake certifications. State hospitals will often provide a certification program to allow evaluators to complete evals on a contract basis, and these are usually legit. Be wary of any other certification that makes you pay for a certificate and says you're then a "board verified forensic examiner in the forensic examiners nation, etc..." I've seen folks then put this "qualification" with their credentials, and it is hilarious. probably not to them later though when they try to say it qualifies them as an expert, only to get subsequently drillled on voire dire.