Good question! I am currently finishing my Ph.D. in psychology.
1. Talk to your professors! Go to their office hours or send them an email asking if there is a time you could meet together to talk about grad school. I am a visiting professor and talk to students about this a lot. Ask them if they have any research projects you could assist with, or if they know of any opportunities to do so. If/when you find research opportunities, ALWAYS show up on time to meetings, let the professor know in advance if you'll be late/can't attend, be polite and respectful, and demonstrate superior responsibility. Make sure your emails are grammatically correct and spelling error free (no text-lingo lol ;-) thx). The more your professors know you, the more willing/able they are to write you letters of rec. Later, ask if they can write a positive one. Although it hasn't happened to me yet, if a non-impressive student (most likely one that has pissed me off, too) asks for a letter of rec and doesn't specify a positive one, professors can and will write a negative one. Generally you will need 3 letters of rec. Depending on the specifications from the school you're applying to, 2 could be from profs and 1 could be from your boss. Also make sure these people get them sent by the deadline. If they are to be mailed in, provide the prof/boss with a preaddressed and stamped envelope. They should seal the envelope, put their signature across the seal, and put clear tape over that. The letter should also be on university letterhead/stationary, but you'll never see that part. Most should be familiar with the process.

2. Take the GRE. Also see if your program requires the Psych subject GRE. In that case you'd take both.

Hope this helps!