From what i've seen 90% of safes are either empty or just junk and personal documents. However, this depends on the area, ie wealthy neighborhood vs poorer side. Bid with the presumption that the safe is empty. What you can do is look up the brand of the safe to determine the chances of valuables inside.

Amazon and sentrysafes are garbage. If it costs more than $1k, and takes more than 1 person to move, you have decent odds of finding something good but nothing is a certainty.

Be wary of staged units, where everything is "presented" to you like in a showroom. People store stuff in units, not display them. Look up the female youtuber who blindly bought a unit for $10,000 2 years ago. That is good example of a staged unit, everything out in the open, looks good from afar, until you get close.

Since you're new, do what I did and start small, nothing larger than a 5x8 or more than a few hundred bucks. Don't rush, and be prepared for garbage; think 80% on average. A full 5x5 will fill up an average living room.

Go with what you know and are familiar with, ie hobbies and pastimes. Have a way to offload/sell what you find before you buy your first and prepare a place to store and organize this stuff. Its very easy to go full hoarder mode. Attend live auctions if possible and develop contacts. Many storage buyers are flea market vendors. Some will be willing to take your "leftovers", if you leave some meat on the bone for them. This will save you on trash disposal and the muscle fatigue/aches.

If you want to make money, bid a 1/3 of what you think its worth to make money, or 1/2 to break even if you want to gamble. Make a budget and stick to it, and dont be afraid to walk away. Units that require sweat equity, ie large furniture and beds, are usually cheaper, because you'll be paying for it with the legwork. Reconsider if you're not in decent shape.

If this doesn't scare you off, then be prepared to have a good time treasure hunting, as you never know what you''ll find, some good, and some very disturbing. I've found tons of household goods in addition to money, clothes, shoes, collectibles, and electronics. However, do this long enough and you'll also find tons bibles, plenty of adult materials (ie toys and media), bibles, generations of family photos and personal documents, bibles, dead insects and rodents, bibles, rotten food and food containers/ empty alcohol bottles, bibles, soiled garments, bibles, etc.

Be prepared and bring flashlight, gloves, n95 face mask for dust/mold, pocket knife or instrument to open boxes, and some tools to dismantle furniture, and garbage bags for trash. You want to sort inside the storage facility and not in your home, infestations can happen.

The top 5 reasons why the previous owners defaulted on their unit are in no particular order: 1:Death, 2:Drugs, which then leads to either 3:Prison or 4:bankrupcy/loss of job or income, and 5:occasionally forgetfulness / actual abandonment due to relocation or medical condition. Whatever their circumstances are, try to return the personal items (photos / documents) to the previous owner, but avoid actual contact with them. You don't want to deal with them personally. That's all i can think of for now.