Welcome to the forum, we recommend reviewing the FAQs on the forum and checking out the Student Experiences section as there is lots of tips/advice/timeline prospectus.
If you haven’t already, you will need to take an Admissions Flight before you can enroll a start date. Admissions Flights are good for 12 months. Once you have a start date, since you’re looking for a start date late Spring/early Summer, you could theoretically start studying for your Airmen Knowledge Tests and completing as many as possible (you are not required to have them done prior to starting, but it’s great to always stay ahead). ATP is a fast-paced program, everything will be laid out Day 1 for what is to be expected. There will be times when you may fall a few days behind and that could be out of anyone’s control, but it’s up to your training center staff to keep you on target.