My experience parallels Yellow Cartman’s and Smerf’s: if it’s a day game during the week, no problem. Evening games are always more problematic. Weekend games, day or night, are most problematic of all. And if you’re in a city with a weak team, but a strong team visits (or a team with a strong player, say one who’s likely headed to the Hall of Fame; e.g., the Reds visit your city - Griffey), then even weekday games can fill up.
I believe every MLB team has relatively inexpensive outfield seats, usually in both left and right field (center field is usually uninhabited in order to give the hitter a better view of every pitched baseball). You can show up a couple hours before game time and buy these ‘cheap seat’ tickets. Again, if a strong team is visiting, you may have to wait longer.
If you see an evening game, then any seat in a new park is going to be a good one. If you see an evening game in an older park (Fenway, Wrigley, Yankee), then you assume a slightly higher risk of sitting behind a support pylon or some other obstruction to your view of the infield. You should ask the ticket agent if your seat has a good view: this is always a good idea for reserved tickets.
All outfield or ‘cheap seat’ tickets will be ok, although you could get stuck behind the foul poles, which are enormously thick (probably 18in. or 47cm in diameter at the base). That’s not usually considered a bad seat, however, since the foul pole often comes into play (there’s a reason for that: hitters are trained to swing only at pitched balls in the strike zone, but pitchers are trained to pitch only at the margins of the strike zone. A well-hit marginal pitch stands a very good chance of landing in a marginally-fair area of the field: the foul pole area). So you could catch some foul balls there, definitely.
It goes without saying that only people sitting in the cheap seats will catch - and get to keep - a home-run ball. If you’re in Wrigley field, it’s tradition to throw any opposing-team player’s home run ball back into the field of play. Don’t try to hit a fielder with it, just throw it into an empty green spot (like this could happen 
- just saying).
If you go to a day game, then your ‘which seat to buy’ strategy will necessarily revolve around the sun. If you don’t mind sitting in the sun, fine, but if you’re in the interior of the country then you should be aware that sitting in the midsummer sun for 2+ hours can be like broiling a steak (you’re the steak). Even in cooler cities, like Boston or San Francisco/Oakland, the sun can get in your eyes, just as it does the players’ eyes, and obstruct your view. So check it out: you don’t want to broil and you definitely want to see what’s happening.
Take some sunscreen, sunglasses, and a good hat (or buy one at the park) no matter what; this is always a good idea during the day.
There are several great food traditions associated with baseball.
One, beer. You’ll be able to buy overpriced beer everywhere, but most stadiums will cut off sales after the 7th inning. There will be several kinds of beer available for sale, and every park has vendors who prowl the seats (you can get it delivered). If you drink more than two beers, your vendor will likely remember you and s/he’ll wander by your seat regularly.
Two, hot dogs. You’ll be able to buy overpriced hot dogs everywhere; eating at least one is de rigeur. Every team has their own version, too. Wrigley in Chicago sells Chicago-style dogs, Yankee sells Coney Island style, you can probably buy a tofu-dog in PacBell park in SF, even. Again, wandering vendors are always around. Some of the newer parks, though - like PacBell in SF - have fantastic outdoor eating areas that are definitely worth getting out of your seat to visit. If you sit in the right-field cheap seats at PacBell, then the SF bay will be at your back (home runs hit over right field and out of the park land in the bay).
Three, peanuts. Always. Your vendor may even try to throw the peanuts to you at your seat; this tradition was begun at Dodger Stadium many years ago by a peanut vendor known as the Peanut Man. Peanut guys will toss the bag of nuts to you, at your seat, often with a bit of razzle-dazzle, maybe even a little trash talk. Owens (the original Peanut Man) was unerringly accurate up to about 10 meters, even throwing behind his back. Not all vendors will toss your peanuts to you, most will walk to the end of your row and your peanuts will be passed down to you.
BTW, baseball fans are used to passing money around. Every vendor will come stand at the end of your row, waiting for your money. You can pass it to the person sitting between you and him, that person will pass it along…don’t worry, it will get to the vendor and everybody knows this routine. This is, btw, one of the reasons baseball is popular: it lets us Americans celebrate the idea that we can still trust our neighbor, even with money. Plus, if you’re sitting in the middle of a row, then when you buy something everybody will get to handle both your stuff and your money. It’s like the pub: instant friends.
Remember to watch for foul balls, some of them are struck very hard. Stick around for the time between the top and bottom of the 7th inning; this is known as the ‘7th inning stretch’ and there’ll be singing (‘Take Me Out to the Ball Game’ is the standard).
Have fun!
