may I suggest your first step is reading this forum thread and following all the included links to the UKBA site.

All New Guide - How to apply for a Spouse/Partner Visa

You should also make searches here in the forum for any areas/topic of concern

Do remember that there is no guarantee that an agent can secure a visa for you.
At the very least I would suggest ONLY using a UK based immigration advisor.

The huge majority of spouse visa documentation is supplied by the Sponsor rather than the applicant.
There's plenty of twists and turns, ins and outs that an agency in the Philippines would surely struggle to well understand. Just my opinion.