These are the biggest sites for custom card creation communities.
Any one of these places would be happy to critique your cards, advise on wording, or chat about the color pie, so come and get involved! There are also a huge number of already completed sets that you can play with! You can find lists of those here (MSE forum) and here (MTGSalvation).
Other great community projects include:
- MTG.Design: A web-based Magic card creator. Much better than Cardsmith, which is to say not garbage..
- Planesculptors: A set database and web drafter. Absolutely essential for finished sets.
- Multiverse: A database for your sets. Useful if outdated.
- Remaking Magic: A custom MTG design podcast. Now non-functional, but worth listening back.
- RoboRosewater: Cards generated by neural network. Puts us all to shame.