I would recommend a class online that costs $12. Its a class on udemy about mixing and mastering and its about 9 hours of videos that shows the basics of compression,mixing vocals, eq, low end, etc. I felt like that greatly helped but I am not gunna lie I watched those videos on youtube by other people that were just as good. The class just has them organized very neat. So you can honestly just find many videos that'll help with mixing on youtube. You mainly need to learn EQ and compression in the most early steps of mixing imo. Also as for the loudness, there is so much you can do. You can duplicate the tracks and pan one to left one to right, layer the instrument, turn up gain then compensate with a limiter or compressor, or you can EQ to bring out the good sounds of that track and etc. You can do so much but first you just need to grasp how mixing works and helps glue a track together while giving each track their own space in the sound. Idk if that helped but just watch vidoes on youtube if you dont wanna pay. Also what I like to do (I dont do it often cuz im not rich) is mix it myself. Then once I feel good with my mix I send that unmixed song to an engineer and compare and see what version I like better. Make sure its a good engineer that wont charge too much. You can find plenty on youtube.