Hi Lana,
the way to find a decent agency in my own experience, is by looking for one that covers a region, for ex, london and airports, manchester, yorkshire etc. these are more likely to be genuine. then meet up with them, if theyre any good they will want to meet you too.
take it from there, if you feel comfortable then go for it, but dont pay any cash upfront until you've been paid first!
good luck!
xxx