Hello my friends, I would like to ask some advice on my current situation. here's the whole drama:

I have started to create a project all by myself, since working with another people, at the moment, is not something that I can do. but I always have problems creating good melodies for the vocals in my songs.

I have a baritone untrained voice, so as you guys can imagine my range is not that great.

Is there some courses about melodies around that I can get to learn some tips or two, even tough I'm not a good singer? I'm more a composer than a singer myself, so I don't wanna let this lack of vocal ability get in my way when it comes to creating songs.
HugoPan Hi, I can be wrong, but I think that you need to test your vocal on different songs just to understand what works good with vocal and what doesn't. 
Sorry, I didn't find detailed courses about your issue, but you can check this video:

Anyway, if you'll overcome these problems, don't forget to share your ideas.
Udjine this is some really good stuff dude, thank you very much. I'll dig more videos of this channel and see what I can use. 
You should learn some songs from singers with your range ( you can research this). If you don't sing any songs you are going to have a hard time inventing good melodies. It's not to say it's impossible, but if you are serious about having good vocals, you need to dive in. It's not just exercises, you need musical content as a base for creating. 

Knowing how to sing is like knowing how to run. Anyone who can walk can run, you do the same thing, only a lot faster. Singing's like that. If you can distinguish musical notes (ie., you aren't tone deaf, like too many people are) and can recreate them with your voice, you can sing. Training and coaching helps make your singing better, just as good coaching improves an athlete's performance. Now, if you can recognize a note you hear, and can match it vocally, then start at the lowest note on your guitar and try to sing it. It'll be too low, but you gotta start somewhere. Then play each half-step up from the open low E string, and attempt to match the note with your voice. You'll find the lowest note you can sing, and the highest. That's your range. And, you'll know where that range is on your guitar as well. Then, you can write melodies using your guitar by sticking within your range. 

If you get training, including using some of the self-help videos on YouTube, you can increase your range in both directions, improve your vocal tone, build sustain so you can sing longer between taking a breath, etc. Just remember, training (including self-training with videos) can make anyone who can sing into a better singer, but it can't make a non-singer into a singer.