theres a lot of good tutorials on youtube, but youre going to want to search more specific things about it.
i would familiarize yourself with a few areas of the program.
-step sequencer
-piano roll
-playlist
the step sequencer is where the instrument channels are, you can drag your desired instruments from the list on the left hand side of the screen onto the channels, add channels etc. this is also where you would add notes and write parts of your song, for drums that are on time with the little boxes, you can just click a box and it will hit that drum when it plays to that beat. to write out notes youre going to want to use the piano roll...
to open that up, right click on the instrument you want to use in the step sequencer and select piano roll. youll see a window pop up, with a piano to the left and a grid lined up with it going to the right, you can resize the window to make it bigger and easier to see what youre doing. in the top right youll see a little box below the "x" to close it, notice the curser turns into an up/down arrow, click and drag to zoom in and out making the grid shorter and longer, i find it easier to write with it zoomed all the way in, you can see better.
now across the top of the piano roll, youll see numbers. each number is a measure. each measure will represent one square in the playlist. now if you just open a new project, your step sequencer will be on pattern 1 in the playlist. each pattern when selected will give you a fresh step sequencer to work with. i tend to write my songs in 2, 4, 6, and 8 measure pieces to keep things easier to arrange, or later rearrange in the playlist.
just play around with it and if you have any questions just ask them here and ill get back to you, also check youtube for your questions, theres a lot of good tutorials already there, and if a picture is worth a thousand words, i guess a video tutorial with someone talking is worth like, a million?