Making music is a big world and if you're new to it it may be best, if you look for someone, that creates your desired soundtrack.
First of all you should make a concept, what role music and sound fx are playing in your game. Maybe think of a melodie or a main concept, that will be associated with your game.
Then think of, what different types of music you need. Take an rpg as an example. There you could have different type and styles of music, depending on your current situation. When the player character is in a fight, it might be appropriate to bring threatening, nerveuse music, that gives the player a hint, that something bad is about to happen
When the player is in a tavern, the music might imitate the happy and drunken mood of the tavern. If you are exploring ancient ruins, a slow and mystical soundtrack might be more appropriate.
I find FallOut: Las Vegas has a good example of well used sound fx and music. Early in the game, you have to get some roots, that grow near a cementery. The music in this area is very quiet and mysterious and sometimes, when you get near some graves, it seems that there are voices mixed to the music, that gives the whole thing a "ghosty" touch. allthough nothing bad happened at the graveyard, I always became a special tense feeling, when I was near the yard...
about the soundeffects: buy a microphone or a pocket recording device and try to find nice sounds. Experiment with materials, go to your kitchen, use your appliances at home, ride with the bus and record your ride, beat your frying pan with your mobile phone
. try to blend out the things you are doing but instead try to imagine, what those sounds could be. be creative. sometimes reversing a sample or applying some effects to it can give it the right touch.
it can be helpful, when you already now what sounds you need. for example, when you need foot step sounds for different materials, write down all the materials you need and then try to find something, that sounds the way, like you think it should sound. walking with high heels on a wooden floor could be imitated by knocking a lighter to a wooden table. Important thing here is, in my opinion, that you have alternatives. Nothing is more boring, than hearing the one single footstep sound over and over again. Make multiple takes and implement it in your game, that the used sounds vary.
I hope this can give you some ideas to start with.