Firstly, don't be afraid if it sucks. First attempts at new forms often suck. Write more of them, and you'll get better.
Secondly, while generally I like breaking the rules, as the saying goes, you should learn them before breaking them. So yes, start off by becoming familiar with the traditional forms and try to stick to one of them (whether it be Shakespearean, Petrarchan, Spenserian, etc.). Do keep in mind that many poets (including the masters) will "break" the rules, and they do this purposefully. Eventually, you can do it, too, but you need to learn the form first before you can start messing with it..
Thirdly, if you think your poem will be dramatically improved by doing something that is against form, go ahead and do that instead. It's your poem. But try to think about why you're doing it, and why you've chosen this form. Ultimately, you want to get to the point where you can think "I think this poem would work best as a sonnet" rather than "I want to write a sonnet, and now I must think of something to write about". That may take a lot of time and many sonnets, though.
Lastly, from a practical perspective, try not to lock yourself into a rhyming scheme that leaves you stretching for obscure words to make it work. Sometimes, English can make this difficult. Sometimes, a bad word choice can throw your whole poem off. But sometimes, creativity can turn an unexpected word into a great choice.
Good luck.
Edit: One more thing. When you're reading a lot of sonnets, try to read contemporary ones too. When you're only exposure to sonnets are poems from four hundred years ago, it can have a weird impact on how you write and approach the form. A lot of people have written sonnets since Petrarch and Shakespeare, and a lot of people have done a lot of new and different things with it since then.