Decide what type of ventriloquist act you're going to have. Consider the type of audience you want to be performing for and tailor your act toward it. An act designed for children will be different than one designed for adults. Your ventriloquist dummy will need to have a race, gender and at least one outfit. Keep those ideas in mind when making your decisions and writing your material.
Acquire the necessary supplies for building your dummy. There are many online retailers that sell kits in the classic Charlie McCarthy style.
Accessorize your ventriloquist dummy. Multiple costumes are a necessity for many performers, as they allow you to change your dummy's character without building an entirely new dummy. You may want to perform at a wedding one day and a kid's birthday party the next. Having a range of outfits will give you more opportunities to perform because you'll be prepared for multiple occasions.