I haven’t tried to lock a checked bag in years; if they have the right to break the lock, I don’t see the point. If there’s anything I particularly can’t bear to have stolen, I put it in my carry-on. I’ve travelled a lot, and never had anything stolen.
Pack your toiletries in your checked bag, so that you don’t have to worry about the gels/liquids restriction. I use (doubled) ziplock freezer bags for this purpose, in case of breaks or spills.
Pack anything sharp (tweezers, nail clippers, corkscrew, knives, etc.) in your checked bag.
I carry a refillable metal water bottle pretty much everywhere anyway, so I bring it through security empty, then fill it from a fountain on the other side. No extra cost.
If a bag is overweight, you will be charged money. Probably a lot. This sort of information varies by airline, and is available on their website – and probably also in the fine print of your itinerary, which you’ll likely have received by email. If you open that file, you can search for “overweight” or some variant, and save your eyes the trouble of scanning for it.
Carry-ons do have a size limit, and experience with this is very variable. Some flight attendants do not care at all if your bag exceeds the size limit; some will force you to check it, depending on their character, mood, and how annoying the last passenger they dealt with was. I’d recommend sticking to the limit. If the plane is very full, they can ask people to check carry-on luggage for that reason, too, although this is fairly rare.
Note that the check-in process has changed a lot over the years, and again, this varies by airport. If you can check in online beforehand, do it. You may otherwise have to deal with an unfamiliar and completely non-intuitive automated process at the airport (some of them are perfectly fine). Many of these require an address where you’ll be staying, so be sure to have that ready. If you are travelling internationally, note that some airports also have you complete half of the customs process using an automated system, and this is hellish if you’ve never done it before and have a short amount of time between flights. In other words… unless you know exactly what you’ll be doing while changing planes, leave yourself enough time to do what’s necessary with as little stress as possible.
I’ve had my carry-on searched a few times by TSA, and it’s never been a big deal. I have a friend who always goes on and on about how friendly the TSA staff are, and to be honest, I’ve generally found them a surly bunch. The articles of clothing that need to be taken off and scanned varies arbitrarily by airport. Computers always need to be removed from the carry-on, but for some reason this doesn’t seem to extend to tablets or hard drives. I tend to just keep my head down and behave like the cattle they want us to be. When herded to a body scanner, I go there and be scanned. I hate all of this, but try not to think about it. I avoid flying through the US if headed anywhere else, so if I’m there, it’s because I have no choice anyway.