Your orchestra needs musicians to play in it. It's best to have all/most of your musicians committed before you start rehearsing for your first concert. Otherwise, your rehearsals will be less productive and more frustrating than they should be.
To some extent, your recruitment efforts will depend on your vision and goals. If you're looking for a higher level of musicianship, you will have to recruit more strategically and carefully than if you are simply inviting everyone who can thinks they can play well enough to keep up. Decide with your conductor ahead of time if you're going to audition everyone or just let everyone in.
Before you start recruiting, make sure you have a concise message to tell about what your orchestra is going to be and place for people to sign up. Your website will be an important place for both displaying and collecting information. A Google form, which you can embed in the website, is a great way to collect information (contact info, instrument, etc.) from potentially-interested musicians.
Musicians tend to know each other, so word of mouth is probably going to be one of the most effective ways to recruit musicians. Once you tell a few musicians, they will tell each other. Start by contacting existing amateur musical groups in your community and surrounding areas and ask them to make an announcement about your new group. Local professional musicians and music teachers might have students who are interested, and you might collect some music students at local colleges and high schools, so let their music departments know. See if you can hang a poster in your local music store. You might also ask if you can hand out fliers or set up a table at other local musical events. Depending on the demographics in your community, an ad in a local newspaper might be effective.
My strategy for recruiting musicians for the RCO was a two-stage process. First, I asked people to sign up (using a Google form on the website) to indicate a general interest in being part of the group (not to make an actual commitment to playing). This resulted in my having 12 flutes and no cellos, but it was at least a start. Then, after some strategic planning meetings with the conductor and some others who were interested in helping to organize, we scheduled the concerts and surveyed the interested musicians to find the optimal rehearsal time slot. After picking a time slot, we had a definitive schedule, so we asked people to make a firm commitment to being in the group (using another Google form). As expected, we lost some people this way (including most of the flutes, thankfully), but it allowed us to do more targeted recruiting and also gave us a body of musicians who could help us recruit. When we held our first rehearsal, we were a little low on strings and heavy on winds, but we had a full orchestra on our hands and all the parts covered. We did not ask anyone to audition.
As you recruit musicians, try to start building the orchestra's organizational leadership. You need to start delegating tasks to capable helpers so you don't get overwhelmed, and you will eventually need a board of directors. Finding a personnel manager early on is immensely helpful during the musician recruitment phase.