First, Establish what it is you're looking for... a DJ, a live band (what genre), a violinist, or a string quartet perhaps perhaps is what you're looking for. Each one will set a different tone for your wedding. After you've decided what kind of music you would like it's time to contact them. You can find hundreds of musicians in your local area, but finding a good/reliable one can be a daunting task.
Once you've made contact, relay the message to the musician(s) what it is you're looking for or the "feel" you want to capture in your ceremony. Most musicians are compliant, for the most part. If it's a DJ you're booking, ask them if they also an MC. The difference is one will be able to communicate with your guests, like a host, while the other one will just play music and barely say a word. If it's a live band ask them if they play other popular hits (cover songs) or if they play their own material. That way you get a better sense of what kind of music you expect to be played at your event, and so do they. Communication is the key during the planning process.
Next find out if they provide their own sound system. Many musicians travel without so much as even a microphone. Believe it or not, many of them expect things to already be set for them by the time they arrive. They just want to plug in and play their instruments. But incase they don't provide their own sound equipment, then you will need to hire someone to put a sound (PA) system in place. Most DJs are self sufficient and will arrive with their own audio equipment ready to load in. But it is also a good idea to find out the musician's needs as far as power to plug in their equipment. You might need to purchase power from the venue for them.
Most musicians will need to get paid in advance before performing at your ceremony. So establish a price that is fair. Tell them what your budget is. If pricing is a concern you should look at what is the most cost effective option for you. For a reception party, a DJ is usually more affordable than a live band for example. For the ceremony you can hire a solo musician such as a violinist or your can have a music playlist with some of your favorite wedding songs ready to play during the ceremony. These are ways to save money and be able to still have some form of musical entertainment during your wedding.
Finally, meet them (or their manager) in person. Find out if they are willing to check out the venue or have a vendors' meeting to make sure everything is on point before the big day. Most musicians are happy to do this. If they are not or "too busy" maybe you should look for someone else to perform at your wedding. There should not be any hiccups before or during the wedding. Set up a sit down meeting to go over things, see if there is anything you want to add or remove. Are you satisfied with the songs they will be performing? Also it's important to let the artist have their freedom to play some of what they believe will bring your party to life. There is a certain level of trust that they have the knowledge and experience and know what to do.
On a final note, Sound For Ceremony can help you find the right live musical entertainment you're looking for along with providing a great quality sound system for your wedding that will have all your guests dancing. It's one less thing for you to worry about between all the immense planning that you're already doing. Just tell us what you're searching for and your budget and we will get the job done.