You’ll find a lot of opinions here – just be careful of absolutes such as those saying “never ride on sidewalks”. We’re all in different environments and some folks have trouble envisioning situations where it may be prudent. Use your judgement – and realize sidewalks have their own set of dangers, particularly at driveways, side streets, and parking lot exits.
Make yourself visible – not just with bright colors and lights but also with lane positioning when in curves or near parked vehicles as you approach cross streets / parking lot exits or similar situations. I use flashing front & rear lights even in day time. I also am extremely cautious if it’s late afternoon and in line with the sun – folks with dirty windshield have a tough time seeing other cars let alone a bicyclist
A mirror – whether bar end or helmet mounted – can help a lot with your situational awareness. Same for not using headphones.
Signal your intentions – and don’t be afraid to take the lane when appropriate.
See if you can find riding groups – facebook groups can be helpful – and many LBS have regular group rides as well. Not only can it be fun – but you’ll also learn of good local routes you can ride whenever you wish.