A lot of producers and execs won't consider a new writer who doesn't have a credit (often referred to as "an unknown quantity") but how does someone get their first credit if no-one is willing to give them a break? It's a catch-22 similar to trying to get an agent (the agent’s not interested until you’ve got some work but it’s incredibly difficult to get work without an agent). So what do you do?
Dedicate yourself to achieving that first credit, wherever you can get it. Forget about big Hollywood sales*, creating your own TV series or nabbing a commission on the next series of Ashes to Ashes. That ain't gonna happen. (It COULD happen but it'll take a great deal of luck and a lot of brilliance on your part.) Instead, think more strategically. Start building your profile from the ground up. Use your spec scripts as red hot samples of your original voice. Convince producers/script editors/execs that you're the next big thing, or at the very least, a writer who deserves some attention.
Write a short film, get someone to make it or make it yourself. Credit! Write a sketch, have it performed somewhere, even if it's your mate's sketch group down the pub. Credit! Write a play. Produce it yourself. Credit! Write a radio play. Submit it to the Beeb, see if they want to commission it or, hell yes, produce it yourself. Credit! All decent ways of building your profile, and giving producers/execs a good reason to consider you as a writer who's a bit different than the rest. The BBC Writers' Academy will only accept submissions from those who have a professional credit. This includes radio/theatre plays. Get in there.
It is possible to get a shot at EastEnders without a credit but you have to impress them in their Shadow Scheme first (and to be considered for this, they need to read & like your sample script, or previous work). Doctors is a good route for new writers. I think there's talk of a Doctors New Writing Scheme but I can't find any details yet (if anyone knows anything, post a link/comment, ta).
If you're thinking children's TV might be an easier way to get a credit, well it can be just as difficult and competitive as prime time drama. But fear not. Look further afield. There's a healthy European market for animation series, and they need lots of writers. MIPCOM and MIPCOM Jnr are two key festivals where the animation movers and shakers hang out. There are opportunities out there. Sometimes you have to take a risk to try to find them.
You're probably feeling eager and frustrated to get your first credit RIGHT NOW. You probably think you're ready, that you can write just as good as the stuff that's on TV, AND that you could do better. Maybe so. But that's from the outside looking in. Once you get your foot in the door, the process of writing a commissioned piece of work becomes a different ball game altogether, and you soon learn whether you can cut it.
One thing is for sure: when your first credit does come along, it will feel right. It will feel earned. You'll be ready to kick some ass. All that hard work will have paid off. My first credit was on Doctors. They had a New Talent Producer at the time, and I pitched and developed ideas with her until Doctors HQ gave us the nod. I cried when they commissioned one of my ideas. It was such a big relief because I was pursuing an 'all or nothing' approach with my writing and I knew it was a significant stepping stone for my career.
Everyone's different. People will get lucky chances. Some will get hard earned breaks. Some will get wrongly ignored or rightly rejected. Rejections pile up for everyone so don't feel too disheartened. They should however become a bit more positive so that you know you're on the right track (Julian Friedmann has some good stuff to say on rave rejections over at his place) . Keep going, keep improving and keep writing and that elusive first credit will eventually come.
* selling your spec script, not enjoying a shopping spree in LA, obviously.