The easiest and most simple way is to click on or make a box around the single unit you want to control. This is inefficient and slow though, there are 3 binds in the hotkeys menu: select hero, select all other units, select all units. These are very useful and I use them frequently on heroes like Brood, PL, any hero i get an illusion rune or manta on. Then there are control groups, an integral gameplay feature of RTS games carried over to Dota 2. Press (by default) a number button to select a group of units bound to that button. To bind a unit you hold control and press the number button you want that unit/s to respond to. I usually bind my hero to 1 so I press ctrl-1 at the start of the game then if I have 1 set of summons like Lycan does I bind them to 2 (you could bind one to 2 and one to 3 but when you resummon the control groups will be gone, this doesn't apply to some heroes but does to others it's a bit buggy). On Beastmaster I have my hero on 1, my boars on 2 and my hawk on 3. If I want to send a hawk to check the rune i'll press w then press 3 and right click the rune spot on the minimap, then if I want to see the rune I'll press 3 twice to center the camera on the hawk then 1 twice to go back to my hero.
If I get an illusion rune on say Tinker, my first thought will be to unbottle it around the 10-15 second mark and then bind one to 2 and one to 3 and then send them to stand next to 2 jungle camps, then I keep an eye on the clock and at 53 seconds (and 55 for certain medium camps) I'll quickly switch to the illusion and stack the camp with it, then go back to my hero.
Illusion heroes like Naga, require efficient use of control groups to micro effectively, I bind my 3 mirror image illusions to 2-3 and 4, my manta illusion to 5 and 6 and any illusion rune illusions I get to 7 and 8. So whenever I use mirror image I'll pres 2 and right click on one camp/lane then 3 and right click another then 4 and another, getting quicker at doing this takes practice but it increases your farm speed a lot.
Meepo is a whole nother boat so I won't talk about him but suffice it to say I usually use all 9 control groups when playing him.