I don't know much about creating your own sounds, but writing music with pre-made stuff works just fine. You will have to learn MML - I think if you look at the manual (an electronic version should be with Petit Computer), it should tell you all of the coding things you need to know about MML.
The immediate problems keeping me from really moving forward with this project is balance and A.I. I want to make the two sides (warriors and wizards) unique in terms of playstyle, but equally disadvantaged against the other type. In the long term, this will be difficult to forsee and fix. A.I. will of course be difficult because reasons.
It has been a long time since I really played SmileBASIC, but it is always in the back of my mind. This is one thing I've thought about a lot - a board game not entirely dissimilar to chess in terms of strategy, but different in every other way. The working title is 'Warriors and Wizards'.
You can change the tempo with T and a number. If many of your notes have the same length, you can instead write L and the length value to set a default length. That means that if you don't write a number after a note, the program will play the note for the length of the value after L.
More about notes...If you want to change the octave notes are playing, you can designate with O (capital o) and a number, or use < to go up an octave and > to go down an octave. V and a number can control how loud the music plays, or ( (open parentheses) to increase and ) (close paranthese to decrease volume.
To use an instrument other than a piano, use @ then a number. The built-in Smile tool is useful for hearing what different instruments sound like. If you want multiple instruments playing together, you can designate channels for them to play on with : (colon).
Letters A - G will play those notes. You can use # for sharps and - for flats. Writing a number after a note designates the length of the note. You can tie note lengths together with &.
A big help is typing MML and looking at the help. It gives you a lot of information on what can be included in an MML string, which is what the program uses to play music. Use BGMSET to save music to play later with BGMPLAY.
4. You can copy code manually, and it is very easy to do a lot because of select+copy. However, if you are just looking to move a file from one project to another, browse projects (in the main menu) allows you to run any program from any project, even if it's not your active project. Then you can save it, and it will be saved to the active project.
Going from basically no coding experience, it took me a couple of months to really start getting the hang of Petit Computer (the predecessor to SmileBasic). A big thing that helped was the help menu, something which was vastly improved in SmileBasic. So long as you're willing to test out new commands and combine them with others, it shouldn't be too difficult.
I hope I didn't come across as mean. I'm just enthusiastic because for me, a big part of having Smilebasic (and Petit Computer before it), is getting to write music. I hope some of what I wrote is helpful as you continue writing MML.