Yesterday, I flushed my toilet and it went down but there was no water pressure. The bowl and tank fills with little water. When I flush it now it either doesn't flush or half way flush. Then I used my shower and as I was taking a shower, the water started rising and then drains reallllly slow. I haven't plunged either one because I'm afraid I will make it worse. But my all my sinks in the house seem to be running and draining fine. So now I don't know what the problem is. What gives? I checked the main line and when I run the sink with water... it appears to flow out fine. I hope there is no obstruction anywhere.

The slow filling toilet and the slow draining shower are unrelated issues. Your shower drain needs rodding, likely clogged with hair and body oil. Now the toilet, does the tank eventually fill to within an inch of the top of the over flow tube?

I agree withBob. You have two separate problems. For the tub drain snake from the overflow, (see image). You will run into a bend about 18 inches down but once past that you only have 10 more feet to put out.
As for that slow filling toilet tan k. What type of ballcock do you have? (see image). Let me know, Tom