After 1NT, 11+ 4-card Major
Most intermediate to advanced players use Stayman in this position. Stayman allows you to find a 8-card Major match if there is one, or revert to NT if there isn't. And whichever is best, it also allows you to find out if you should bid up to Game.
But, if you haven't agreed with your partner to use Stayman, then here's what you do. . .
Balanced
With a 4-card Major, but not a 5-card one, in many cases you are going to be balanced.
Since you have no 5+ card majors, your distribution is probably 4-4-3-2, or 4-3-3-3, or 4-3-5-2, for example. In that case just go and look at the last < page.
Unbalanced
With the rarer distributions, 4-4-5-1, 4-4-6-0, 4-3-6-1 you are obviously unbalanced. In that case just go and look on the next > page