Tiberian Sun was the best of the C&C games in my opinion. Just configure a game using a preset map or generate your own, and jump in. Set the game speed option to something lower to begin with until you get the hang of what to build and when, etc.
Deploy your MCV and build a base, building refineries (and harvesters) to harvest the tiberium (blue is worth more than green, and explodes when shot). Explore with the few units you start with and find where the nearby tiberium is and where the enemy is coming from - you can destroy bridges with rockets, tank shells etc., and repair them with engineers. Tiberium is harmful so don't walk your troops across it unless they are tiberium-enfused already (e.g. cyborgs, which I think heal in tiberium). War factories allow you to build basic vehicles, barracks or hand of nod allow training of basic troops. Increase your tech level by building a radar station and subsequently whatever the other tech building is called, to unlock your highest tier weapons, including the strongest units of which you can only have one at any time (mammoth mk. II, or cyborg commando, etc.). The tech level you specify when you configure the game determines which units and buildings you are allowed to create - 10 allows the strongest units and the superweapons (ion cannon, or cluster missile). Engineers are consumed to capture enemy buildings or restore your own buildings to full health - you keep any vehicles docked at a building if you capture it.
Combat is pretty straight forward: you can usually tell whether a unit is strong against infantry or armour. From memory I think you do everything with the left mouse button, selecting units, telling them to attack/move/deploy etc. The single player campaign eases you in and has some good cutscenes with hammy acting - the series is famous for this.
Like all RTS games if you want to get competitive you need to know the hotkeys, so you don't have to do everything with the mouse. If you're just playing for fun you can get by with the most useful ones: Ctrl+number to assign a team; x (I think) is scatter, so your troops don't all get run over at once; h to move the camera back to base. Just searching online will get you a list of hotkeys, forums discussing tactics, etc. I don't know anything about competitive play, but I do have a fond memory of playing this game on my first PC, which had only 500MB hard disk space. I could either have Tiberian Sun or Encarta Encyclopedia installed. Those were the days...
