First off pick a league or nationality to base your team around for example.
If you choose the Liga BBVA then you will want to select only players from that league to fill your Starting 11 and Subs bench. If you're going with nationality then for example you would search for players that are English so you would look for players only from England but the league wouldn't matter.
Another key thing is decide on wether or not you want to play as a Bronze team, Silver or Gold. As people prefer different level of challenges, Bronze makes it easier and cheaper to afford and run a team with the ability to always turn a profit. Silver is moderate when it comes to team building it may take 3-5 matches to find yourself opening a new pack but my tip would be never open packs unless absolutely essential. Gold is the hardest difficulty to play at is it costs alot of coins to fund your team and contracts on a consistent period but offers some of the best players available.
Chemistry and formation, this is so important to the basis of your success other than your skills at the game. Choose a formation, 4-1-2-1-2, 4-3-3, 4-4-2 and so on choose one from the many options and start buying players in that formation. Why? It allows for better chemistry between players especially if they're all from the same league, team or country.