New players, listen up! I just want to bring a hotkey trick to people's attention because even I've been playing for a while and somehow missed this until not too long ago. If you play as Zerg this is particularly important but it affects everyone: appending to a control group!
We all know ctrl + # assigns units to a control group (and if you didn't, you will be flying now!) That makes the game playable. Once you get rolling though you need even more command over groups and especially newly built units. You will find it frustrating that you can't find time to send re-enforcements. However, by having units selected if you press shift + # it will append that to the control group number you want them in. So say you usually do ctrl+1 to make a task force assigned to group 1. If you have a bunch of guys coming out of your base and need to quickly add them to your existing group you can ctrl-click/double click to select them all then shift+1 to add them to your existing control group!
This becomes incredibly handy when paired with awesome zerg hotkeying, since you can ctrl group units after you've selected your larva and told them to spawn into a unit. So, as Zerg:
1) Highlight hatch(es)
2) S for select larvae
3) Press Z three times to spawn 3 zerglings
4)press ctrl+1 to instantly assign that build to control group 1 (another trick all new zerg players need to know!)
5) Later, go back to your hatch, do step 1-3 but this time press shift + 1 to add your newly spawning units to control group 1.
6) When moving your outgoing party on the map, keep pressing 1 then right click somewhere. While you are out and fighting you will have re-enforcements already grouped up in your selection AND rallying to where you are!
7) profit.
Couple this with hotkeying your army buildings (barracks, gateways, hatcheries) and it becomes a tide-turning force for you. I always assign those to number 4 on my keyboard so in battle I just press 4 then do the above steps and I always have re-enforcements to topple my enemy.