It's free to visit, but if you want to reserve tickets in advance, there are various fees. I got online to do that but as the fees kept adding up, I decided that it couldn't be that crowded on a weekday morning in November so I'd skip the advance reservation. If we couldn't get in, we'd just get tickets for another day. When we got there, we were able to get right in with no wait so I'd skip the advance tickets if you're going at a less busy time of year.
You can walk through both the Peterson House and Ford's Theater at your own pace and there are good museums and displays in both, along with people there to answer your questions. Most were NPS employees who were very helpful, but there was also an actor at Ford's Theater who was great. There was a lot to see at both sites and they are fascinating parts of US and DC history.