"OK" - somebody might say - "I am now a (Mere, non-denominational) Christian. What next? How to choose a denomination?"
If there is one you find wholly true and wholly trust, then there is no problem.
But what when you find the modern landscape of Christian churches to contain none to which you can whole-heartedly commit yourself?
You might find a specific church, a specific congregation or group, but almost certainly there will still be a requirement to commit to a denomination and that denomination may well have elements you honestly regard as false, overemphasized, gaps, or corruption.
Yet, if at all possible, you need to join and attend a church - at some level of frequency and activity.
Suggestion: Treat a denomination like a political party or a scientific theory or a friend or any worldly entity - don't expect perfection in all things (and don't behave as if such an entity exists).
But ask yourself, what is non-negotiable, and work from that.
Then consider your own motivation, what brought you to Christianity - the problem, the obsession, the need to escape some thing, or the yearning for some state...
That may be a clue as to which of the real denominations might provide the best focus and emphasis.
Then see what is available, what is accessible with what frequency - and develop your personal devotional life.
Note: These are intended as suggestions to move forwards a step or two, aimed at those feeling stuck in a position of being theoretically Christian - what then? This is not any kind of blueprint, recipe or formula.