Boston is awesome but it generally has a heavy amount of traffic. You will want to give yourself plenty of time to get there. Make sure you give extra time if you need to find somewhere to park and also to get through security.
One of the things I do when figuring out timing is I go to Google Maps on desktop (mobile doesn’t do this). I ask for directions from my house to the Garden (as an example) and then I alter the date and expected arrival time and it will tell you when to leave by. Then you might need to account for things like Feasts in the North End, walks (like the Avon walk) or the Boston Marathon (in April) or Red Sox games, etc.
Where you are going to TD, you won’t be impacted a ton by Red Sox games because it is in another part of town. But always give yourself extra wiggle room time wise to get there (traffic, parking, security).
Edit: I agree with some people below too – you will need to buy tickets in advance more than likely.
Good luck! 🙂