Set your tactics and training. Various suggested routines and details of how to set your own are discussed elsewhere on the site. Have a good long look at your existing staff. If they are of a reasonable caliber for the standard of the league you are in then make sure that they have a decent contract from your point of view. Do not sack your existing staff until the new guys accept your offer of a contract.
Some clubs have no players at all.
If not then you need to get into the transfer market very quickly. You should ensure that the positions determined by your chosen tactic are covered. If those slots are not covered or cover available via the transfer market then you will have to give serious thought to a tactical change. There is no point in using a tactic that requires left and right defensive midfielders if you don't have any. While you may think that you have players capable of covering those particular positions the effectiveness of the tactic could well be reduced.