i should start with TFCD, test shoots, etc, but how much is a "decent portfolio"?
First, you have to decide what kind of model you want to be. As a rule of thumb, a "decent" portfolio comprises of between 25 to 40 "spreads". A "spread" is 2 pages, which can contain 1 image each, or 1 image on 2 facing pages. Why 25-40? Anything under 25 is not "strong" enough and anything more than 40 will bore the client.
A standard industry size (regardless of region. If you intend to grow out of the usual local event type modeling, disregard this paragraph) is 9"x12". Meanwhile, the 8Rs and super 8Rs are not going to cut it. Of course, if you are just contend to have an "album" of pictures, disregard this paragraph.
Next, decide on what sort of portfolio you want. Different portfolio will get you different kind of jobs, and it is not uncommon for a model to have more than 2 books for different auditions. For example, if you are interested in commercial jobs (they pay the most per assignment after high fashion runway), you want to put images which are like advertisement. If you are interested to do editorial, than go for lifestyle kind of images. Regardless of "types", always includes some beauty shots (i.e. close ups of your face).
Next, finding a photographer is only 1/4 of the job. To have a successful photoshoot (at least to build your portfolio), you also need to book an MUA and a hairstylist. Depending on your looks, you might just be able to convince some to work with you on a TFP basis. Last but not least, you need a wardrobe stylist (which is sorely lacking in this forum). The wardrobe stylist is the one who puts all elements of clothing and accessories into a cohesive look/image. 90% of photographers here knows nothing about fashion.
To be paid or not to be paid? It all depends. If you are starting out, then opt for a TFP as I cannot see why a photog will be willing to pay you to shoot. Furthermore, if a photographer pays you to model for him/her, then he/she is not obligated to give you any images because "you are paid to model". So if you want to build a portfolio, going with TFP will be the most cost effective option.
Other legal issues: You are underage, regardless your potential as a model. As such, you are required by law to have a parent/legal guardian to transact on your behalf, particularly a model release. A model release is a document where you literally sign all your rights away, i.e. the photographer will have all rights to the images and if he decides to use your likeness to make a profit, you do not have a claim to that share of the profit, neither do you have a right to prohibit him/her to publish your image, with or without your permission. Of course, if you are paid, then it is natural that you sign a model release (that is what a photographer will pay you for, the future and/or possible use of your image for publicity and/or commercial gain. If it is a TFP/TFCD, it is not mandatory for you (in your case, your parent) to sign any release. You can, however, sign a release but limiting the photographer to use it for commercial purposes, but not as part of his portfolio, or publicity campaign.
I think I covered the basic essentials. If you have further question, feel free to PM me. Good Luck.