It's no good applying to a modelling agency when you want to be an actor, just like it's no good applying to an agency that solely represents musical theatre actors when you want to be getting screen work! Research and agency before you apply and make sure that you look like a good fit for each other!
Make sure you have everything ready when sending off applications. It sucks when an agency will only take people with certain skills sets or perhaps will only take Spotlight registered performers but there is no point applying if they have already specified their application criteria and you don't fit. It's a Waste of yours and their time. Have what you need (headshots, showreel) ready to go and then apply!
So many actors start out in there careers and before they have any experience they start applying to agencies. It's really important you get credits behind you before applying to Agencies / Casting Companies - remember an agent needs to see that you are able to land work before taking you on, when you land roles, you will be helping to pay their bills!