Freelancer-turned-agency owner here!

Step 1 before anything is make sure you have a portfolio up and running. Whether it's client work or fun projects or case studies, put it all up there. Make sure your skillset is clear to visitors, and make sure it's easy for people to contact you. Some people are happy to fill out a contact form, some prefer to email you directly, some prefer calling you (if you want to offer that as an option).

Getting your first clients:
I found my first non-friend/family clients many many years ago via social media. I saw some "I need freelance dev help" tweets from a couple other freelancers who had too much work on their hands. That's actually how I got my start with Shopify, which is now what my agency focuses 100% of our efforts on. I'm also a member of some development/freelance Facebook groups (such as Freelance to Freedom). People often post leads there for work that doesn't fit within their niche or it's just not something that can/want to take on.

What to do, what not to do:

• Don't over-promise what you can't do. If it's a similar skill and you just haven't had the opportunity to dig into a specific facet of your preferred development language, it's fine to take on the project if you know it's something you can do based on your current skills. But don't say you can code an iOS app for a client if you've never done any iOS development before.
• Don't do anything for free. Your time is valuable. The client wouldn't be giving away their product for free, and neither should you. If you're still building up your portfolio, you can discount your rates if you want so you can get some paying clients under your belt.
• Always, always, always send a contract. Never do any work before you have a signed contract and a down payment. How much you want to set your down payment for is entirely up to you. On some of our projects we require 100% up front, and on others we'll accept 40%.
• Be realistic about your time frame. If you think something will take you a week to code, tell the client it'll take two. Things always come up that will throw off your schedule. It's better to deliver earlier than expected than to have to push out a launch date.

Client communication:
We always send a questionnaire to the client before scheduling a call with them to make sure we're a good fit for them and they're a good fit for us. Whether you want to schedule a phone call, have a video conference call via Zoom/Skype/Hangouts, or meet in person is up to you, but we always go under the assumption that any in-person meeting will take up half of our workday. I honestly don't like having in-person meetings because of this. But regardless, it's always a good idea to have some sort of discussion with them off of email to see if their personality and communication style are a good fit for how you prefer to work.

Establishing your rates:
My favorite topic! Everyone undervalues themselves. It takes just one potential client who says you're too expensive to make you second guess your rates. I've been in this business for many, many years and it's still something I struggle with.

Shopify put out an article about two years ago that does a really good job of covering you should set your rates without giving explicit numbers. It's difficult to say "You should charge X" without knowing where you're located, what type of work you're doing, what your skill level is, and the many other factors that go into pricing a project. It's definitely important to take your market into consideration, as rates are going to differ from place to place. (My rates [I'm American] are often higher compared to what a lot of Europeans charge.) And another word of advice - always continue to increase your rates with each project. Another agency owner once told me that if a client is willing to pay $6,000, they're probably willing to pay $7,000. If they're willing to pay $12,000, they're probably willing to pay $14,000, and so on. Lastly, never reduce your rates for a client without removing something from the scope.

I hope this helps!