How To Learn To Say No Without Feeling Guilty?

I have a habit or maybe one of my weakness where whenever I said "NO" to someone request to help them especially to my friends, the feeling of guilt will slowly take over in me.

SITUATION 1 
FRIEND : Hey want to go out tomorrow?
ME : *feeling a little tired and lazy* Erm never mind lah
FRIEND : Eh why? Come lah holiday mah. Many people even better hangout later
ME : *fake laugh* No lah not feeling well. Next time bah okay
FRIEND : Oh okay rest well
ME : *2 minutes later** Ugh I should have say yes just now*

SITUATION 2
FRIEND : Can you help me do this please? I really don't know what it means.
ME : Here are some examples you can refer. Try see first then maybe you will have idea later.
FRIEND : *5 minutes later* I really don't know what I am doing lah. Yo help me check mine lah and help me correct it.
ME : Haah?? Erm okay okay *leave friend work like that and didn't check*

SITUATION 3
FRIEND : Hey later long semester break can I put my stuff at your place?
FRIEND 2 : Yaya. I have thing want to put at your place also. Hostel store got limits leh.
ME : *I DONT WANNA HELP YOUUUU. MY HOUSE GOT NO PLACE TO PUT YOUR STUFF LAAAAH!* Oh okay then.

In the end I will just agree to help them and in the end I'm the one who always end up with overload works and responsibility. Plus some MIGHT thought I'm a bit "SOMBONG".  It is not that I don't want to help you but what people said? Tak mampu or I don't think I can.