Scratch

hi im new to scratch but still know more than freinds at school lol. Anyway they made me promise to make them a halo platform game. my problem is that i have made walls see but the sprite just walks through them. i want it to stop, not turn around, not bounce, just stop but nothing works. does anyone know how to do this without long strings of variables and numbers because they make my head explode. SOMEONE PLZ HELP BEFORE I GO INSANE!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!