If you have an android phone, use StreetComplete
It turns this chore into a game!
Community for the city of Vancouver, BC
You can go on openstreetmap.org, login and click edit at the top right. Then the in-browser id-editor will open and you can straighten out the building lines. There's even a feature to 'square' areas to make them have 90° angles
Yes, there are so many things that can be added to OpenStreetMap and we would love more help! I would start by asking: what do you use OSM for? or, what would you like to see more of in OSM?
I am a frequent OSM contributor in Vancouver. Personally I like to find a niche and work on it for a few weeks before I lose interest and find something else to map. I use the iD editor on openstreetmap.org.
I also like to pull out my phone when I am in a new neighbourhood or shopping area to see if there is anything I can add with StreetComplete or Every Door.
There are so many possible ways to contribute to OSM. I don't think the map can every be complete.
The wiki is an important guide for learning how to map more features. wiki.openstreetmap.org
Another helpful way to contribute is to upload GPS-tagged through Mapillary.
There are tools like Osmose which help identify QA issues https://osmose.openstreetmap.fr
Hit me up if you have any questions!
If you want to fix building lines, I think JOSM is the best tool. You do need a few hours to get acquainted with it.
If you want to contribute with points of interest, apps like StreetComplete are the way to go. The most active OSM community is on Discord, I think.