Only use the Scout, AWP, and Deagle in competitive matchmaking. Your teammates will love you, I promise.
But srsly, in competitive matchmaking make sure to buy armor and, if you're on CT side, buy a defuse kit whenever possible.
Stock up on smoke grenades, incendiary/molotovs, flashbangs, and HE grenades whenever possible.
Don't spend all of your money when buying guns and stuff at the beginning of the round (unless its the last round of the half). Personal and team economy is important.
Don't ever buy an auto sniper unless its the last round of the half or the other team buys one first and you know for a fact you will not die while using it.
Learn your map call-outs. Some might be different from what's shown in the upper corner, but just listen to what others call areas of the map. Call-outs are very important in letting your team know where the other team is.
Play with a headset and a relatively high volume. Sound is key in CSGO. You can hear footsteps (unless the person is walking), gun reloads, and scope-ins with snipers if the person is close enough.
Get acquainted with the fact that you may never leave the Silver rank group in competitive.
Crouching and less movement help accuracy.
Don't just play Dust II.