How to fix my 1991 ford explorer xlt?

I need help on how to replaced the lifter
how to do a tune up
how to do a oil change
how to fix my a/c since its blowing hot air
and how to fix my oxygen sensor… I know this is alot but i don't have that kind of money to fix my car. I don't know anything about cars. Please help i'm a mother of five and i'm doing this for them and my schooling to provide a better future for my family.

Go read a book. Preferably one on auto repair and maintenance.

Sorry but trying to explain this stuff to someone with no tools or experience is a waste of time.
HOWEVER start doing searches on google for videos - you should find most of these in pretty good detail.
Tune up, oil change are the easiest.
O2 sensor is a tough one in many cases - but doable with the right tools.
You are NOT going to be able to do the lifter - that is a job better left to a mechanic.
AC needs a lot of special tools that are one-use and it probably has a leak and the freon is very expensive - so just deal with the heat until you have a 500-1500 saved.

Most auto supply stores sell repair manuals. Chilton's is the best.