Is it OK to use a template to build a portfolio, or should I make my own custom template?

This is all up to you. Being your first portfolio I say go with a template and work on customizing it with your own touch. Every time you make a site you'll be learning countless new lessons in the process even when using a template.

After that try your hand at your own from scratch. I've probably made about 50 portfolio designs over the last few years (a perfectionist is never satisfied) and whether I used a template or not I've learned a lot. I started with templates and now make my own from scratch (some of which become a template I'll use on future projects.)

So, just get designing whichever method you feel comfortable with! It's all about learning and practicing at this point.

Good luck :)

Honestly if I was you I would create one from scratch. Visualize a design and go about creating it. It'll will be a lot tougher but it will pay off the in the end. If you have any problems post on here, google or post on stack overflow.

I think the best part of templates is their design not necessarily their code.

I'm of the belief you should know what every line of code does it's always harder to debug code you didn't write. If you can't yet implement a feature you'd like to have on a design.

Then I'd urge patience, Rome wasn't built in a day.