A professionally designed flier (not the printing, just the design) can cost thousands of dollars. Not only is that beyond the cost that many beginning groups can afford, it may produce a piece of marketing material that is not in concert with the stage of your career. In other words, you don't want your flier to make it look like the ensemble would cost more than what your target concert presenter would want to pay. Concert presenters have a good idea how much they should pay for groups at different career levels. If your flier looks like you might expect to earn $40,000 per concert, and the presenter has never heard of you, you will probably get the gig. Erring in the opposite direction is equally bad.
Take a college course in graphic design, collect materials from ensembles you admire, learn some of the computer software (free to several hundred dollars: I use In Design and Photoshop, part of the Adobe Master Collection, as well as many other programs), start a relationship with a local printing company, and begin the learning curve to producing effective marketing materials.
A great place to start is your local printing company. Find someone there who will guide you as to what they will need from you in order to print your materials. They will help by telling you what computer formats they need from you, what kinds of papers and inks to use, and even give you design ideas.