Below is a number of links to videos and websites that will be useful references to help you modify or repair your garment. If you know of another, please add a comment.
How to unpick a seam
Removing Overlocking the easy way
Quick tutorial to sew on a button
How to sew on press studs (snaps)
How to hand stitch an invisible hem
How to tie a knot (persevere and practice this, it’s so much easier than other methods when you get the hang of it!):
How to unpick seams for clothing alterations:
Removing Overlocking the easy way:
Quick tutorial to sew on a button:
How to sew on press studs (snaps):
Another method to sew on press studs (snaps):
How to hand stitch an invisible hem:
Search Youtube for lots of other video tutorials on all aspects of hand and machine sewing.
If you are really new to sewing, try starting here.