However the tank may still have to be established. Planning ahead is the best way to deal with that. Plants however, with a little water in the tied bag, can be left bagged - assuming they are getting a fair amount of diffused light in the house. Don't leave them in the bag or box and don't cook them in the sunlight. ;)
The plants may be rinsed off in a gentle flow under the tap. Long, tough plants like Java fern, Val and sword plants may permit a careful sliding of fingers along their blades to remove any algae or detritus. Anything which doesn't belong should be pulled off if possible. Visible snails can be picked. Damaged or dead leaf material can be scissored off.
A small plastic wash basin is handy for this - but it should be dedicated to the plants and fish, not dishes. An extra tank (ha!) or bowl is also fine, assuming that the previous residents didn't just die because of the creeping crud.
But what of pests that can't be seen?
Those annoying little plastic pots and that "wool" can be discarded. Roots should be gently disengaged. Later, should you so desire, they could be placed in small, clean clay pots. A little aquarium plant fertilizer, or even soil, can be placed in the bottom. The gravel will be gently poured along side the plant(s). Some put a 1/2" layer of sand in on top of the soil and then place the gravel.
Maybe the unpotted plants should first get soaked in a potassium permanganate solution to get rid of snails and eggs and disease organisms. It is supposed to be a pretty good oxidizing agent and hopefully will do in any unwanted hitchhikers. Residuals of the chemical in their new home will probably not be a problem.
If one can not find that stuff, one might pull out the alum. Many, not all, drug stores still carry it. (Remember canker sores and grandpa's cure - worse than the affliction?) A level teaspoon of alum goes into a gallon widemouth pickle jar (or drum bowl) with a couple of ounces of really hot water to dissolve. When that is stirred and dissolved, cooler water is added until the jar is full and roughly room temperature. The plants will be added and left for 40 minutes. They will then be rinsed several times to get rid of the alum.
Alum is an astringent. I recall an individual, perhaps a bit troubled, who dropped snails in to watch them shrivel up. I want that for disease organisms. One will really rinse and rinse because one doesn't want that stuff in a tank with their guppies.
Be gentle planting. Curl a hole in the gravel, place the plant in and gently cascade the gravel along side. Assuming it is affordable, get plenty of plants. They will do better and, as a group, more effectively compete with algae for nutrients.
There's a lot more to aquascaping. But for now, turn on the light. Jump back quickly. ;)