This is a difficult one. I was once told to be on spotlight, but then spotlight were very picky, and demanded you have at least two lead roles in feature films. But how can you even get to be an extra, if no one will represent you. To get some credibility, there are many adverts on here, that require people for adverts, some or many are for charity, but at least that will give you some experience and something to put on your résumé. Some acting workshops, get their students bit parts to help build their portfolio. I myself have played SANTA for 40 yrs now, and irrespective of that, was declined to act as SANTA. I had already put myself forward to be in a NON PAID role for an ad for charity, and donated my time and services. Then, Santa and his elf, were taken ill, and I was phoned, and asked if I was available. - I know I was only a seat filler, but still it was a PAID role, and the Freebie along with the Santa role, went on my résumé. Then I put the company that hired me on behalf of their client, as my representative, as they then took my details and ´registered´ me on their books.
It is good to look at which companies are advertising, and checking their websites, as the smaller ones, don´t mind you having many companies representing you, as one company may specialize in high profile clients like: BBC, itv, 20th Century Fox, Netflix etc., while others supply extras and models only, and others do performing arts e.g. people who walk around as Storm Troopers, Peppa Pig, or Frosty the Snowman etc.
So put your name forward, and just register with them on-line, and have them contact you. It will happen, and if you have a few freebies, put them on your CV, just don´t say it was a freebie, as they might not classify it as a CREDIT. But let´s face it, acting is acting, whether you are doing it for charity or not. Some people started as stagehands sweeping the floor. Others got involved doing work for free. MY advice: Just go for it, and May the fourth be with you.