An agent is really a must if a writer expects to get treated well in the deal-making process, wants to sustain a career (one that builds exponentially) and wants to deal with reputable companies and producers.
Carving out a career is very very difficult. It's highly competitive and very political (matters most usually out of the hands of the writer). A skilled agent knows how to manipulate and handle and overcome the treacherous system.
Landing a manager is more believable for new writers (they are more open to new talent). It is, IMO, the best route to go. Managers work with writers, help them pick the right material, introduce them to poroducers, execs and agents. But they cannot make the deal. You need an attorney for that. However, agents do make the deals - and that is their primary focus. (And the primary difference. Both get 10%.)
The best agents are at the bigger agencies - because they work in teams and can package projects. Smaller agencies don't have that kind of clout - but can be good launching pads for new writers. Smaller agencies can only grow writers so big (then the writer moves on).
Ultimately, it is less about the agency and more about the agent.
As suggested above, there is all sorts of material out there to hone you into representation. But for most new writers without some sort of buzz or heat (your film got lots of hype at a reputable film fest or your script is a Nicholl finalist), you'll have to find the right concept to break into the business.
It is unlikely for a new writer with no heat to land representation with a script about an 18th century bookbinder who falls in love with a courtesan. Managers and agents are looking for something they can sell. If they can't see a place for the script in the market, it's a pass.
So, the advance work in a search of a rep begins with writing saleable material.
Good luck.
