There are many, many opinions on the subject of teambuilding and the best place for that is the community forum - especially for the regular question of "What is the best team from my troops? I have ..."
Some general rules are as follows (exceptions can be made to basically all of them though):
Try to have all six colors represented (or five if not six). Its more efficient and prevents some utterly useless moves.
Try not to have too many troops of the same color/s, or the ones near the back won't be able to get mana as quickly.
Mana generator troops can speed up teams dramatically - chief among them being gem creators (i.e. Dryad, Acolyte), color transformers (i.e. Alchemist, Valkyrie), and exploder troops (i.e. Herdmaster, Carnex).
Make use of your Hero - especially early on. The ability to have a bulky troop with multiple color options via hero weapons should not be underestimated, this is an easy way to satisfy many criteria in this list.
Remember troop bonuses for kingdom/s, type/s and colors when building teams.
Remember any traits that are or can be unlocked.
Try to have some kind of synergy between your troops (the most basic level of synergy is the color spread thing mentioned above). For instance, if a troop's ability deals double damage to frozen enemies, put something else in the team that can freeze enemies. If a troop has a great ability but is very expensive to cast (i.e. most Legendary and Mythic troops), stock the team full of mana generators to get the appropriate mana more rapidly.
Order the team properly - if a team has a troop that works best in the front, make sure its in the front. If there's a fragile troop (underleveled, or just is), place it somewhere it won't be hit. With troops that share a color or colors, make sure the priority troop is in front of the less important one/s.
Pick the right banner. This might be difficult earlier when the banners aren't all available. Try to unlock the +2 banners first for general "make this color better" options (they're from Karakoth, Whitehelm, Forest of Thorns, Khaziel, Stormheim and Pridelands). Once more banners are usable, mix things up to see what banner works best for a team - if two or three matches will fill up a Red troop even if it has +1 Red instead of +2 Red, consider dropping the +2 Red banner for a mixed banner.
Have more than one team - have the right team for the right match. If your main team can't break through a certain composition no matter what you try, the solution might be an alternate team entirely. Also helps to keep things interesting.
Ultimately, you want to make teams that you enjoy using. Once you really know better, optimize for speed and consistency.
Any questions not answered here should be directed to the community forum or in-game chat channels.