Most important question to ask oneself prior to starting: What do I feel like doing?
It can be as simple as sticking wings and cannons onto Bumblebee and be done, or as complex as completely scratchbuilding Unicron out of styrene. What you need is an idea to do something, which is frankly quite tough to advice on
But just to get a quickstart on things, perhaps you can try making simple mods to old unwanted figures you already own; parts swap with other unwanted figues, stick a couple of gundam weapons, repaint with color of choice. The only limit is your imagination, so go wild!