Hi Romanticstudent,
This question is right up my alley.
I'm a current B Psych student who has done TSP units in first and second year. And if I am correct, I was one of the students that was involved in the 3D project last year that falseenthusiasm mentioned. Falseenthusiasm is absolutely correct about the flexibility of taking 3rd year courses in 2nd year (I took 3rd year advanced stats this year, so I am 100% sure of that). However, I am in the science stream.
You will get an invitation as long as you have made the requirements (which I am sure you have, if they sent you any information). Even if you do not, as long as you are close, I would highly advise sending the TSP supervisor for psychology an email. I am not sure who it is this year, as I believe the supervisor has changed.
I am not sure about the art aspect. It is true that it is scientific, but psychology is taught within that framework, so I don't think it'd hurt to at least try the TSP project out. For 1st year, only a project is offered (maybe changed this year?) - this is a group presentation. You will be guided by a 3rd year, so you don't have to worry too much. It's also quite fun and you'll enjoy it (and you will learn how psychology really works, not the mind-reading mumbo jumbo that you often get from TV and media).
The process of enrolling is quite simple for psychology. You go to TSP psychology supervisor, fill out a form, go to faculty of science to get it enrolled and you are done! But first, wait for the email for TSP invitation.
And falseenthusiasm is correct. TSP will not affect your progress in BPSych - treat it as an elective. Of course, it is (usually) advantageous to get more experience in research as it will most definitely help your Honours year, h/w it will not affect your later studies in psychology, if you decide to pursue them. Though you have to be careful with your subject selection, if you are in the arts stream - that means you need to have a arts major alongside psychology (which is a science major). It probably means you have to overload the TSP unit on top of your semester courses - check that out! Though I am sure if you are invited to the TSP, you should be able to do a 3cp project ontop of usual 24cp load (I did this in my 1st semester 1st year, overloaded TSP project).
If you have any questions, feel free to ask!