Go to www.youtube/bas1sta and watch his videos. Really great if you want to get better.
also if you play 1v1 I would suggest Brute or anders. ALWAYS go for the hooks on hook based maps. Go harras, or rush like you call it, the opponent with brute squads and once you have set him far enough behind go banshees. As anders make warthogs in the beginning , harras the opponent and take hooks. Than spam tanks/hornets/gremlins/gauss , you should adapt to whatever the enemy is building.
If you play 2v2 i would suggest cutter or arby. With cutter you should expand right at the start if there is an open base (which is he case in most 2v2 maps, thats why cutter is good in 2v2 ). Work on your economy first , so a lot of supply pads and than just make scorpions. When arby , go temple second and harass the enemy covenant/stop expansions/stop vehicle depots/ stop airpads etc etc. Than depending on how well you harrased you can either go banshees or vampires (if the enemy has air units).
So to sum it up : unsc main task is ground units a.k.a tanks whilst the covenant takes care of the air war.
remember these are just basic tips which should get you to trueskill around 35 if done correctly.
Add me on xbox live if you want to play some 1v1 scrims or 2v2 :US DEPT JEM555
I am not the best but my trueskills are , 1v1 : 42 2v2 : 40 2v2 (random people) : 41