The first thing you will need to do is keeping a basic tactic your mind. What makes the difference between a good manager and top-rated manager is the ability to adjust his chosen tactic to his team's squad. However almost anywone playing CM has a couple of standard tactics and we all prefer to adjust the squad to the tactic, buying the appropriate players instead of adjusting the tactic to our players, and just doing some fine tuning at a later stage.

When i say choosing a tactic i do not of course mean that you need ot know which exact formation the team is gonna use. However you should know if you are gonna play with 3 Defenders or a 5-3-2. Are you gonna use 3 strikers or 4-5-1. What i generally do is take a quick look over the squad. I use the position and side filters to see certain categories of players and then i sort them by value. I take a quick look at the top players in certain categories, especially the DCs, the wingbackes and the strikers. If i have good wingbacks, i migh move towards a 4-4-2 formation or towards a 2-5-3 with DML-DMR (sort of 4-3-3 with the DL-DR moved forward to DM position) If i have at least 3 good DCs, i am inclined to play 3-5-2. The most important thing however are the strikers, because a good striker is always more expensive than, say, a good DC. This means that you are less likely to be able to buy 3 good attackers, and you will have to be based largely in your team's own forces in the attacking line. So if i have just a couple of attackers, with the one being an FC rather than SC, i will probably play a 4-5-1 with the AMC advancing forward to an attacking position. 3 strikers means probably some 3-4-2 or 4-4-2 (2 in the main team squad and at least 1 substitute, plus one striker i mighta buy as an extra) and then if i have at least 4 decent FCs and SCs i am more likely to go on to some formation with 3 attackers!

Important things you will need to have in mind before picking a tactic:

1. Wingers and wingbacks are always more expensive compared to central midfilelders and defenders. So in case you are out of funds you might prefer to choose a formation in which you will use more DCs, and MCs(DMC, MC or AMC) than DRs, DLs, or MR-MLs. In such a case i generally do not use wingbacks and most offen revert to some 3-5-2 formation.
2. Although in real life football a team has to have players on the sidelines of the pitch, it is quite true that in CM formations in which the players are positioned centrally , with many "layers" of players between the opposite attack and your GK seem to be woirking better. This means that for example in case you are using 3-5-2 it is more preferable to be placing your midfielders as DMC/MC-MC-MC/AMC thus producing 3 "layers" of players than as 5 MCs in a line.
3. You will need to have one first-team player and one substitute in each position. If you have just 2 strikers, for example, you can not play 4-4-2. There will certainly come a time when your attackers will be injuredor unavailable or simply exhausted. Always keep this in mind while designing your formation!
4. Many people tend to go on the net and download formations someone else has made. I really can not understand why somebody should do this. Noone guaranties that a formation made by someone else is better than your own. Most important making and testing your own formation is much fun and downloading a ready one just spoils the game. however in case you still want ot download one go to our tactics bible. For instructions on how to use a downloaded tactic please refer here
5. Those are just basic guidelines to help you get started. If you really want to master the tactics issues in the CM and be named a real manager, you will also have to deal with aspects like passig style, with and without ball movement and giving separate instructions to each player. More can be found at our tactics bible