All good advice here.
Since our tastes vary, starting out, and while it's hard to tell which blend might suit your tastes, I suggest doing some homework.
If you know, for example, you enjoy virginia blends, take the time to note which of those blends might appeal to you based on what you read here on BoB and elsewhere, and then the reviews on tobaccoreviews.com.
I suggest taking notes on each blend you're drawn to and why. If you enjoy more than one type of blend, say, the occasional aromatic, create a separate "Bucket List" delineating each blend within those types.
Soon, you'll have enough information to cherry pick one or more ounces and/or tins of some blends that you feel will work well for you. As you smoke them, add to your impressions to your notes.
While doing this, you're developing a list of your favorites and at the same time, gathering important details on each blend you can use to compare a favorite to other blends you may be thinking of trying out.
If you feel particularly gregarious, you can add a "curve ball" once in a while. My last "gamble" was El Rincon de la Pipa No. 1 from 4 Noggins -- it was new to me, had some good reviews, and I was looking for a flavorful yet mild English, so I bought a couple ounces and liked it, so I've since bought more of it.
As for how long an open tin stays fresh, in my experience, it's about 3-4 weeks -- at least around here where the ambient humidity is average. If I'm not going to finish a tin in my rotation within a month, I'll jar the rest. If buying a baggie of bulk, it should be jarred right away, or throw a pouch moistener coin, charged with distilled water, in the bag (you can also extend the freshness of an open tin this way too).
Ball screw-top canning jars are fine right out of the case. I remember reading another puffer, with a scientific background state emphatically that new jars are actually cleaner than if the same new jars were washed with dish soap and dried and dish soap can have a detrimental effect on the "good" microbes required for proper aging. When I want to reuse jars, I wash them in the dishwasher on the sanitary (hottest) cycle, then heat dried. I inspect the lids and if they're fine, I reuse those as well; if in doubt, I replace it -- lids are cheap and sold separately wherever the Ball jars are sold.
I use a Sharpie pen to "label" the blend and MM/YY on the lid. A drop of acetone (nail polish remover) on a piece of paper towel will wipe that off when the jar is empty. The filled jars go back in the 12 jar cardboard case they came in, a Shaprie black marker is used to I.D. what's in the case, and the full case is stacked with the others in my cellar (a dark, cool and dry walk-in closet).
I fill the jars, fairly well packed (I use the standard 8 oz. jars, which hold 2-3 ounces of tobacco, depending on the blend) and leave 1/2 inch of air space at the top -- tobacco needs air to age properly. I then screw on the lid tightly and return to the newly packed jars after a week to retighten if necessary (this is because the microbes begin to do their thing right away and the process will "suck" the lid more tightly onto the glass jar -- you can actually hear the "ping" noise when that happens, it's kinda cool).
Good luck, enjoy, and remember -- like most things worthwhile, the more you put into it, the more you'll get out of it
