Can I write tied notes instead of half notes? tied dotted quarter + an eighth note? If not how can I write this better?
12/8 is essentially 4 lots of 3 quavers, and needs to be written out in that pattern. True, the sum total for one bar is 12 quavers, or 6 crotchets, if you like, but it represents compound, not simple time, as 4/4 is.
As it stands, assuming that's one whole bar, then you have the equivalent total note value, but in 12/8, it isn't written thus.
With nothing else to go on, the time signature for what's there could easily be 3/2. But perhaps you mean 'how can I write 3 equal notes in a bar of 12/8?' Then, you could simply make them three triplets, which most readers would understand.
As with any notation, you want to make clear where the main beats begin. In your example, it's unclear where beats 2, 3, and 4 are; they're somewhere hidden in the middle of each of those notes.
As you've guessed, you'll need to clarify this with ties and note values other than half notes. Consider the notation below:
This notation clearly shows where beats 2, 3, and 4 are by giving them each their own dedicated notehead. (You don't need to have the "2", "3", and "4" above the staff; that's just for explanatory purposes.) By showing where these beats lie, you help guide your performers through the measure.
I should, however, show one other possibility, albeit a much less commonly used one. If the tempo is fast enough (so that the 12/8 really occurs in 2 as opposed to 4), it could be helpful to clarify them as triplets:
But this is best saved for only certain occasions. If the tempo is such that the 12/8 is felt in four, then most musicians would perform the above example far more accurately than they would this one.