If you were an alien race, and wanted to enslave earths population. Wouldn't the first action be to get rid off all old and sick humans?

No. That would leave only the fit and strong to fight us off. 👽

No, not at all. Never. The first action I would want to take if I were an alien race would be to immediately cure the old & sick humans. This would be piece of cake for an aliens to accomplish, since the alien race would be way more advanced than earthlings, and would surely have a 100% effective cure for even the most serious & incurable human diseases (such as covid19, cancer, HIV, diabetes, etc.) & conditions (such as old age & disabilities).

No.

Give "credit" and 

sell drugs, alcohol and tobacco.

No. In comments you've made to others you're getting rid of the old and sick because the young and strong make the best slaves. And because the young and strong are still no match for the aliens might that they're also those who'd be the fighter is irrelevant. That makes the old and sick so irrelevant that you wouldn't waste time nor effort killing them

First I would get rid of the fighters.

No. Why would I get rid of a segment of the population that posed no real threat? I'd target all the military of Earth first. 

No. Unless I lacked a brain I wouldn't be able to replace fact with fiction. No offence. :)

No? The strongest, actually. First, weaken.