Here are some useful links for understanding MIPS ISA:

http://www.cse.iitd.ernet.in/~sak/courses/cdp/cards/r3000-isa.html  --MIPS R3000 ISA Summary

http://fourier.eng.hmc.edu/e85/lectures/MIPS_machine_language/      --Machine language for MIPS ISA

A detailed programming reference can be found on the website of MIPS Technology Co.:
https://www.mips.com/products/product-materials/processor/mips-architecture/

Also, Professor James Larus provided SPIM: a MIPS32 Simulator, a very useful tool which allows users to execute MIPS assembly code:
http://pages.cs.wisc.edu/~larus/spim.html

These links plus the "Green Card" on the textbook, you should be able to solve all problems in homework 1.

Still have questions? Come and talk to me in discussion, during my office hour or send me email (gshi2@wisc.edu)

-- Guangyu