Step 1: Buy tickets either online or at the gate of AT&T Park (home of the San Francisco Giants).
Step 2: Arrive to the game with a pen/sharpie and the item you would like signed about 2 hours early, so you can get in the stadium as soon as the gates open and be there during batting practice.
Step 3: Go to the main deck and look to see if anyone is signing, if someone is, go to the spot where they are signing. If no one is signing (that you can see) head towards the left corner of the field (section 135) and try to get as close to the field as you can.
Step 4: There is almost always a player warming up in this section. Politely yell out their name and ask for an autograph. Usually the player will turn around and either walk over and start signing or give a 'one minute' signal with their hand. Do not get impatient, the best thing you can do is wait until they come over to sign.
Step 5: Another great place to go for autographs is towards the dugout. Towards the end of batting practice multiple players will often sign for fans.
Things to take note of:
Be polite. The worst thing you can do is be obnoxious. Always say thank you after you get your item signed.
If you are unfamiliar with the Giants roster it isn't necessarily a bad idea to print one out. This way you can call out to players who you do not recognize.
Black pen is the best thing to have a baseball signed with. Sharpie is best for a hat, glove, card, or picture.
Be patient. You likely will not receive an autograph instantly, the best thing you can do is wait, although it may seem like a good idea to go somewhere else, waiting a little while is the best way to get an autograph.
Do not expect to get more than one autograph, especially if it is your first time getting autographs at a Giants game. Manage your expectations. You will often probably receive more than one autograph, but it is best to pleasantly surprised than disappointed.