Hi, I recently started playing the original StarCraft to get ready for StarCraft 2. I have absolutely no StarCraft experience, hell i have no RTS experience.
I have the speed all the way up and it still takes me about 1-2 hours  to finish a mission.
I'm currently doing the Tarren campaign. I usually spam SCVs in the beginning of the mission, build any structures i think i need, then build a shit ton of units and just go from objective to objective until my units die; rinse and repeat.
 
Can you give me some tips to considerably improve my game?

Same camp here dude. I say, if you are playing only single player, just follow tutorials, see what works, and use each faction in the best way possible.  
 
Now, if you're going online, look up some youtube videos and/or guides to get you familiar with how people play, what build orders you need, etc.

Well if your gonna play competitively online then it might be a good idea to start looking at replays and videos online. There are tons of tutorials and unit specs to look at online, and they really help improve your game. But if you are just playing the game casually and want to gradually improve your abilities then just play the game at your own pace and try out new strategies frequently. A good rule of thumb is never rely on a single unit producing building, especially in the late game. And if you find yourself running into even match-ups then upgrade appropriately. Figure out what units counter other units, and improve your usage of hot-keys.There really is no absolute right way to play starcraft, but no matter how you decide to play it can be very fun.