As BAS said, the books are effectively "match-makers".
Person A (you) likes the New York Yankees and wants to bet $100
Person B likes the Boston Red-Sox and wants to bet $100
The book charges the loser of the bet $10 in order to be the broker in-between person A and person B. That is called the Vig, Vigorish, Juice, et al.
That $10 also covers the fact that there may be no Person B, and the book-maker needs act as Person B if he is unable to match the bet. In the long run, ultimately the book becomes Person B as most wagers don't come in with each side having equal action.
Now, as nojuice9 pointed out above, if you are willing to play the underdog in a game, you can actually wager $100 and win $110, $120, $150, even $200 or more.
When looking at lines, you may see NY Yankees -$150 / Boston RedSox +$140. In this case, if you wager $100 on Boston, you would win $140, however, if you want to wager on the Yankees, you'll need to bet $150 to win $100. In this case, you're playing the "percieved odds" of the yankees winning/losing and "vig", which is the $10 difference between $140 and $150. Generally speaking, the higher the perceived odds, the greater the spread (vig) between the bets. Example, if you could see The New York Yankees -$220 / Boston Red Sox +$180.
Only God knows the "true odds" of one team beating the other. The books price and payout gives you the "perceived odds" - generally, the book factors into the equation what people "think" about a team's capabilities of beating another team. Teams like the Yankees / Lakers / Cowboys, generally have a higher than actual line as they are "public" teams with lots of followers who will blindly bet on "their team".
Before you ever make any bets, as a newbie you should read this forum for at least a year and have a season or two of tracking with fake money to see if there's any real chance of you making money. 95% of the people who gamble lose. Now, if you want to do it for recreational purposes only, betting $5.00 here and there, I don't see much harm in it, but if you really want to do anything with this sit back, read each sub-forum here on each sport you're looking to play and learn, learn learn.
Good luck.