I'm thinking on creating a IPB forum.
Which version should I get? (preferably free) And does it has the Portal thing?

Help me please!!!

Your best bet is to go and buy the Perpetual License which includes lifetime upgrades and 1 year support. You can, if your low on cash, go with the Yearly license which is $70 rather than $185. The hitch is that you don't get any support or upgrades after your year expires. You do get to continue to run your board though if you choose not to renew your yearly license. I have a lifetime license (in essence, same as the perpetual) and I have been very pleased with the support and stability of IPB. It is by far the best forum system on the web.

You won't go wrong

Does version 1.3.1 has the Portal feature?

I don't think 1.3.1 has the Portal, but there is a modification for it.

I suggest 2.1 when it's released. You'll need to buy a license to use any version of IPB thou.