With Street Fighter 4, mixing up you attacks is the key to winning. Its all about mind-games. If your opponent is playing aggresively, don't charge to meet him. Hang back and counter, which is especially easy if he likes to jump. Once you've knocked him down, start messing with him. Don't just go on the offense and try to hit him; instead, wait until he makes a wrong move then capitalize.
For example, your brother is playing as Ken and you are Ryu. A simple trick is to get a knockdown (using crouching fierce or EX shoryuken if he is attacking). Once there, move in close, hug his body. His instinct will be to do a Shoryuken to counter you, but you'll have already guessed his move and will be blocking, giving you a free hit. Next time, he might get wise to your strategy and just stand, in which case you can grab him, or do a low-risk attack like a jab to test his block and beat out any follow up moves he may be getting ready to throw. If the throw works, repeat. If you connect the jab, follow into an easy combo (jab jab medium shoryuken is a good one). If he blocks, dash back to throw him for a loop, or close for the throw.
You have to keep stuff like this in your head all the time, but luckily most of it is pretty self-explanatory once you get into a fight. Other than that, memorize your combos, to bee able to get the most damage possible out of any given opening. And learn the match-ups. Some characters use similar strategies to one another, but no two are entirely alike. Relying on one fit-all strategy will get you beaten by anyone with experience. Also learn how to zone properly. Zoning is, in layman's terms, knowing how to keep your opponent out of your personal space. Many fierce attacks are very slow, but have great range and priority, which makes them ideal to use when your opponent is coming at you, forcing them to stay away or get hit.
One last piece of advice, that should be self-evident but I see ignored all too often. REMEMBER TO BLOCK. Don't constantly attack, and remember no matter what your friend says there is no shame in running if the momentum is against you. No pro in the world would stand toe to toe with Zangief if he could make room and throw fireballs, and neither should you. Lastly, just relax. If you get tense or frustrated, you'll make mistakes. The game may seem fast, but if you see behind the graphics you'll realize its just a complicated game of paper-rock-scissors.
