Ok, so actual tips:
Constantly produce scvs and mules. Really at the lower levels it is far more important that you have more units than it is particularly what units they are (within reason). To do this you need to be absolutely maximising your economy at all times. This cannot be stressed enough! Even in the heat of battle carry on producing them (hotkeys help with this a lot). I'd recommend practising against computers to get this into your head.
Linked to the above, always aim to be broke

. Unspent resources are wasted resources, so always spend them. Find you have 400 minerals? Expand. Are you producing from every unit producing building and still resources are building up? Build more. Excess gas? Upgrades. Anything, just spend it. It's at this stage where you can find out for yourself how many barracks/factories/starports you can support off one base.
Never get supply blocked. Fairly obvious, and it happens to all of us, but if it is happening regularly then you are screwing yourself over.
Never queue units, you're basically just having unspent resources by another name.