I have finally finished my first book, but I'm not quite sure where to look for editing or publishing. I'm interested in traditional publishing, but from what I've heard and read, I need an agency for that and I don't want to risk losing my book to an untrustworthy agency. Are there any agencies that have been used by other authors and proven trustworthy? Where should I look for editors, and later on, publishers?
You can start the editing process on your own. Reread what you wrote and give print copies to trusted Beta Readers (I used to give my drafts to family members in addition to Christmas gifts until they outgrew the target audience). Beta Readers are good for both the grammar and general story structure as they can give you a few good "first time reader" audience reactions, something which you'll never be able to see (you know what's going to happen all the time when writing. They don't know it when reading). Plus they can point out syntax and descriptive errors (maybe you have complicated movement in a scene that you explained well to yourself... but needs more clarity to others). For your own editing, I've heard and have done "Read backwards one step at a time)" is great for grammar editing by the writer (I.E. If the story is "See Spot. See Spot Run. Run, Spot, Run." then you would edit "Run, Spot Run. See Spot Run. See Spot." This helps the writer to focus purely on the story, since the reversed event flow distance yourself from being story critical.
As for finding agents, there should be plenty of signs to look out for to keep the terrible ones away from you and the good ones will tell you what policies are in place to stop this (Since they don't like thieves ruining their profession.). Shop around, ask questions, take notes, compare, ask about other agents you're looking at and ones that don't. Sometimes they specialize in genres so they might only deal in fantasy... but they know a guy who will take your scifi.
The SFWA's "Writer Beware" project has some pages about how to find an agent, adn how to spot a bad agent. See particularly "LITERARY AGENTS" and "HOW TO FIND A (REAL!) LITERARY AGENT". These mention various red flags, particularly charging reading fees and referrals to paid editing services, lack of a verifiable track record, and unprofessional practices.
These pages are, in my view, worth looking at. There is much otehr good advice under the 'Writer Beware" umbrella.
This is mostly focused on agents, since the editing part is pretty well covered. For agents, go to a site like Query Tracker. You can get the free account, and that's likely fine for a first-time author. I think other, equally reputable sites may have started having free accounts as well. There will be comments on agents, and I'd start looking over agents and agencies well before sending any queries. Visit their websites, read their bios and the things they are looking for CAREFULLY, because there's a lot of subtext about what they want. You can do a lot of screening and even pick favorites/do not query lists.