Sentence Help ASAP

Hi,

I have some sentences that I need help with. If someone could help me correct them and explain to me what is wrong with each sentence. Thanks.

1. Lincoln is the most greatest American president.

2. I went there before two years.

3. The classroom is cleam, it looks very good.

4. If I was a little smarter, I'd have gone to Harvard.

5. I like sports since I was little.

6. She should have went there yesterday, not today.

7. In my opinion, the difficult in computer is there are many things you forget about it.

8. At that day, mary waited for him.

9. This garbage that they throw it.

10. She should have saved money as many as she can.

11. If he want from her to forget his mistake he must have invited her to dinner.

12. If anyone wants to sell any products, he should do many researches and gets all the information possibilities.

Again, I need to have the problems of each sentence explained and the corrections. Thank you.