Giving occasional lessons to people i noticed a few things i always explain, so i tried to put together all the general advice that anyone could use ranging from ~1000 to 1700 (and probably more?) rating.
Nevertheless, you wont learn anything from any kind of advice unless you start putting effort into it. The point of this thread is to give people guidelines on what to focus on in their games to start improving. Basically, a list of mistakes every lower rated player does - and with starting to get rid of those hopefully you will become a better player.
And now, lets keep it short
1. Your first and utmost goal is to get your half of the map. That is the very basic thing and if you cant get a hold of mexes and reclaim on your side - you effectively just cant compete with your opponent. Learn the map, learn the ways to take and defend your expansions. Otherwise no map control, no eco, no units, no victory.
2. Eco efficiency. Very crucial and unforgiving early game, a bit less but still important later. Learn BOs from top players or make yours. Your goal is to spend your eco as effective as you can with no stall or (minimum) floating resources. The later in the game you can maintain (near) perfect eco balance, the better player you are.
3. Other efficiency. First, learn why BOs work for top players and what they get from them. Better start, better ending. Usually its utilizing nearby reclaim and mass/hydro points to make as much factories and as early as possible. Which leads to faster getting units on the field to raid and deny map control from your opponent.
4. T2/T3 transition. Generally the best time to make one is when you are already going steady with your map control and have enough units on field to play with. If you dont, go back to previous points and work on those Dont try to rush higher tech units (there are always exceptions though, but remember this is just general advice), first make sure you have enough production to hold your side of the map. And on the contrary, dont overbuild and sit on lower tech tier too much. Only reason to do so is below.
5. When to eco up? Same as before, first condition to meet is maintaining map control. Second condition is making sure your opponent cant do substantial damage with the units he has. Still cant kill your army and break through? Ecowhore some more
6. Fighting and raiding. Make use of your BO to get those first units as fast as you can killing enemy engies and mexes. You should remember though that raiding becomes less efficient the longer the game goes. If during first minutes sacrifing your raiding groups for denying expansions is worth it, later the unit cost will outweight the losses. Losing 20+ t1 tanks for killing 4 t1 mexes is a bad idea. Losing a bunch of t2 tanks for same t1 mexes is disgustingly terrible idea. Never send your units to mindlessly attack enemy positions. If you see you cant break through and deal some real damage - dont send your units in the first place. Eco up, tech up, and look for another way you can deal the damage.