Isometric is generally a 1:2 ratio of pixels, like so:

Then, you want to separate different angles of walls, this can most easily be done by establishing a light-source, like so:

Also, you'll want to keep in mind when you're doing iso, or any kind of scene like this, you might want to keep things 'even'. Like if you have stripes on the wall, you might want to make the width of the wall divisible by the width of the strips. This way, you'll keep the pattern on the wall even, and it won't suddenly come to a stop like yours has at the corner, it looks very odd. The best way to do this is to make a small portion of the wall and draw the stripes or what not on the wall, treat this small bit of portion like a tile, then tile it like...:

This method isn't full-proof, but it's a good step in the right direction to balancing out the patterns on walls. Either way, you might want to touch up any mistakes after you're done laying it out.
The most important part is to make sure you maintain the 1:2 pixel ratio lines. They are what makes iso-art so clean and help create the false perspective. I mean... there is also 1:1 isometric art using diagonal lines, but that's rare and few pull it off, even those who do it doesn't always create the fake perspective that isometric is meant for.
And to note, it will always seem like the room is 'inverted' and such. This is a trick of the eyes and can not be fixed until more detail and items are added to fulfill your expectations of an interior, and not a flipped cube.