What To Look For In Players
I'm a firm believer that you should not limit your squad to specific type of player. I believe that variety and diversity can be a wonderful thing to have in your squad, giving you many new dimensions to play with both tactically and individually. It allows you to have a set of different weapons in your arsenal to choose from, and reduces limitations of capabilities that may be imposed on your squad for whatever reason. That is just my opinion, however.
Traditionally, Determination has always been seen as an important attribute for any player in FM. A player with a high value in this particular attribute is more likely to develop his skills. However, do be aware that this attribute can be altered through mentoring and is not set in stone for most players. One point I would make though is to not limit your search for players to those who have a high Determination attribute. While it is a very good thing to have, players with high Determination may not be available in the quality your require. I have made several signings with low Determination in this save alone, purely out of necessity. It would be foolish to believe that all players must require high Determination.
The other attributes to look for in a player can differ depending on what role you are looking to fill. I suggest you set up Filters in your Scouting tab to whittle down the choice of players depending on their specific attribute values. While I use my own set of Filters which I alter (sometimes drastically) to match what I am looking for, fmscout.com's lead Twitch streamer Jamie provided a set of his own Filters which you may download and use at your own leisure. It is important to note that the filters only mark out which attributes to look at for a player in a specific role, while you should set the values for these attributes yourself, depending on the level of player you are looking for and which attributes you value more highly than others. You'll get the hang of using Filters quite quickly, I'm constantly making new ones for myself to keep my options open.
I cannot stress this point enough. Only sign players who will be of use to you, unless you sign them on non-contracts where you won't be paying them any sort of regular wage. Having been a member of this community for many years, I have witnessed countless players buying a ridiculous amount of players every single window. How many of those players actually play a part for them? Very few. Make sure that the players you are bringing in have the quality to make an impact for your team, whether that be as a regular or as a backup. There is no point in signing players to rot away in your squad.
PATIENCE is key when looking to bring in new players. Do not rush into making signings, this may force errors in judgement that would not ordinarily happen. Sometimes you may have to wait a while for the right player to become available. For example, Jürgen Klopp & Liverpool waited half a season to sign their primary defensive target, Virgil van Dijk, last season (in real life). Was he worth the wait? Yes, without a doubt. They have since recorded their own best defensive record in the opening 13 league games of a season EVER.
The most important thing you can take from this section is that it is up to you to balance the pros and cons of each decision you make in the market. You must decide whether one players' capabilities outweighs his deficiencies, whether his price & wage demands are worth the transfer, or (sometimes) if you should hold off on making a signing in the hope that a more ideal player becomes available at a later date. This kind of decision making will come with experience, you just have to show faith in your own ability as a manager.