study tactics, either on a site already suggested like chesstempo.com, or even on chess.com. You should successfully complete at least 10 tactics in a row per day. if you mess up, start over. If you've completed 9 in a row several times and keep messing up on the 10th, you can cut yourself a break at first, but in the future step away from the tactics trainer at the 10th problem and come back when your mind is fresh.
play a lot of chess games. Some people will say blitz games are really bad for your chess, and if you aren't studying your tactics then I would say this is mostly true, however, if you study tactics you can get a lot out of playing a few of these games. Mostly, though, I would suggest playing 3 15+minute games per day and analyzing them with a stronger player (maybe even the one you've just played) or computer software, like fritz.
Play stronger players! It will blow up your ego to defeat weaker players, but you will not learn as much as often. In order to improve you must learn to play against people better than you. Often they will use tricky tactics, like discovered attacks and overloading defenders. You will start to see these tactics more often before they are used against you and you will even spot them in games against your opponents!
After you've become familiar with tactics, the bread and butter of a chess game, you should learn at least one, but to start with no more than two, openings for each color. It is important to familiarize yourself with an opening's different variations and pawn structures. It is much better to do this than to thrust your pieces in the center randomly each time. It will help you coordinate your pieces better if you recognize patterns from previous games, and even see when your opponent who might be less familiar with the opening is falling into a trap. Your game will improve drastically as you become comfortable with your opening; remember, though, that the opening comes after tactics. The opening will give you a strong positional and strategic understanding of your game, but if you can't recognize faulty pins or other clever trap tactics, you will probably fall for the same thing over and over again.
After you find an opening for each color you like (mine are the torre/trompowsky for white and sicilian dragon/caro-kann for black) study some games of the masters who use those openings on chessgames.com. Start with the games of those who win, so you get the basic idea of how to be successful with the opening, and then after you feel like you have a handle on that, play a few games of your own trying to emulate the basic opening patterns and go back and look at some of the games of those who've lost. If they lost on middle game blunders or tricky tactics you might still be able to get something from the opening, but overall you'll find that the players you play will probably not play the best moves, at first, so you will develop your own understanding of the positions that arise and will make the openings your own.
These are the best tips I can offer you. If you do your tactics every day, play 3 good games, win or lose, and develop your pieces with a consistent plan each game, you WILL improve. It may not be overnight, and at first when learning an opening you will probably lose many games, but you will start to see the patterns and recognize positions where you might have made a mistake in a previous game, and you will have the chance to correct them. More than anything, understand that your opponent will likely make mistakes and it is up to you to find them. The longer one has to think in the game the easier it will be for someone trying to improve to spot a good move, so 15+ minutes only at first is best, while you learn, and then when you've comfortable with about 10 common variations of the first 10-15 moves of an opening you can start playing 5 minute blitz. I don't recommend bullet chess until you're around 1300-1500 (even then it will kill your chess unless you are really good at quick tactical games and know some good opening traps).
Let me know if you have any more questions. Most of this advice I've received from NMs and IMs I've met before and it has all helped greatly, plus I teach kids chess for a living and I've seen tangible results with this advice at that level.
Also, as an aside, the three basic rules I love to teach kids about the opening are
a) attack/develop towards(occupy) the center b) develop all your pieces before you attack; move each piece only once in the first 8 moves, start with middle pawns, knights and bishops c) castle the king and connect the rooks asap.
With these basic opening principles you should be fine against beginner players.
Chess is also a huge strain on the mental faculties and playing or training when you haven't slept, eaten or have just spent a large amount of energy on an activity like working, studying or exercising can be detrimental (however, exercising is key to playing better), so play when you're well rested and not hungry for best results. Mostly, you should be having fun. If you start to get frustrated and find yourself making bad moves quickly, you are likely playing emotionally, which is almost never a good thing for a beginner. Emotions destroy chess. Intuition improves with skill, but never with emotions. take a break for a day or two if you start to see your chess skills decline. When you come back, you might find you're excited to play again and you'll see that you have actually retained a lot of the knowledge you've gained, but you won't be so frustrated anymore.