Seriously. I'm not tone-deaf, but I can't seem to hit the right pitch most of the time. What can I do?
If you can tell you're not hitting the right pitch, you're off to a good start ;)
It sounds like you need to spend some time on rudimentary arpeggios , scales and so on.
Finding out what your range is would allow you to know when you're trying to sing out of it. The closer you are to the end of your range the harder it is to stay in pitch.
If there are any songs you're perticularly good at that may also be a place to begin investigating. What is it about that song, key or style that makes it so comfortable?
There are a ton of beginner singer exercises that you can find on youtube and the rest of the web, so i wont try to advise on that, but learning how to sing a basic major scale would probably help.
There's a great article about opera singers who stop being able to hit certain notes not because their voices are ruined, but because they are no longer able to hear certain pitches, so hearing a note in your head goes a long way to being able to sing it. For this i would reccommend just spending some time listening to the material you want to learn, and really focusing in on the voice as an instrument.
It's not directly in your question but it is implied. With or without an instructor an important point in progressing is to SET GOALS. They don't need to be lofty, and in the beginning i find it better to only set short and medium term goals, nothing over 3 months. Write them down, set a date and then work to get there. Your overall reason for singing will dictate what your goals are, and when you commit to completing a goal by writing it down, your mind sets to work on how to solve problems and get there. Set the success condition as explicitly as you can so that you know when you've got there.
Also, check out some tutorials on YouTube, the voice is a fascinating instrument as it is a direct projection of your phisiology and your mental state. It is the performer's most naked instrument and can be extremely diverse and complex. I suggest you find a forum or local group of singers to learn with, the beginner problems can seem impossibly daunting, and often are easily solved by someone that can recognise them.