u must read the tutorial of modelsim it will give u a good start, u dont need to know every thing in modelsim at first since u can learn new things with time but ill summarize important points for beginners:
-u must create a library to contain 2 files of .vhdl extension(ur testbench,ur design)
-u compile these files and the compiled output will be also in the library automatically.
-its better to create a project then create a library inside it .
-try to build a very simple design(AND gate logic for example then a flip-flop) to minimize error possibilities.
-if error appeared during compiling open the source of error and read the error message carfully(somtimes it points dirctly to the cause of error)
-start running the simulation.
-remember to initialise the clock to begin either from 0 or one or a statment lik this wont work correctly" clock <= not clock after 20ns" because ur design when first loaded it will use X if no logic is specified for the clock
as i said u must read the tutorial its very simple and ask again if u have further questions.
good luck.