first, you can use library.
or, you should implement custom icons into drawable folder and put code below.
MarkerOptions markeropt = new MarkerOptions();
markeropt.snippet("Snippet");
markeropt.icon(BitmapDescriptorFactory.fromResource(R.drawable.custom_icon));
this solution would be helpful!
here is a link's snippet.
Bitmap.Config conf = Bitmap.Config.ARGB_8888;
Bitmap bmp = Bitmap.createBitmap(200, 50, conf);
Canvas canvas = new Canvas(bmp);
canvas.drawText("TEXT", 0, 50, paint); // paint defines the text color, stroke width, size
mMap.addMarker(new MarkerOptions()
.position(clickedPosition)
//.icon(BitmapDescriptorFactory.fromResource(R.drawable.marker2))
.icon(BitmapDescriptorFactory.fromBitmap(bmp))
.anchor(0.5f, 1)
);