The first thing to do is eradicate all the poison ivy. Do not use fire to burn it off! All parts of poison ivy, Toxicodendron (Rhus) radicans, are poisonous. Even smoke and ash from the burning plants can carry the toxins (urushiol) to exposed skin, and breathing the fumes can cause life-threatening pulmonary reactions.
Herbicides labeled for poison ivy can be used. Follow label directions for rate and time of application. Depending on the herbicide several applications may be needed. Some resprouting of poison ivy may occur several months later. Watch this area for at least a year and repeat the treatment as needed. If possible, waiting for one growing season to observe the life cycles and bloom times of existing plants may reveal many desirable plants you wish to retain in your woodland garden. Targeting the poison ivy specifically will leave these established wildflowers and other desirable plants undisturbed.
During this time you can take a soil sample of the area to determine any amendments that may be needed. After the poison ivy and other desirables have been eliminated you are ready to plan and plant your new garden. There are many resources that can guide you in the selection of the plants for your area. Contact your local Extension office or a local garden center or nurseryman for adivce.