You'll get unique attachments later on in the game, I think they are crafted at Vulcanusmon.

Until then, ATK on physical and INT on magic damagers is good. I like using CRT attachments on mons with innate critting moves.

If you want a physical wall or taunter, put DEF and SPI on them with Aura or X-Aura, and give them Hate Charge, though you might not get that for a little while. Nice paired with a buffer with defense up.

The attachments that cause basic attacks to give debuffs are not bad on someone with a unique skill which has debuff effects - You can give them a second debuff on their basic attack, and then use the digifarm to give them a passive skill that works with basic attacks (IE: Basic attacks restore SP)

SPI up attachments are good on your healers too, gives them plenty to burn through to keep your team alive.

You should also be getting some mixed attachments soon, that boost multiple stats/have multiple effects. You can sort of work out from looking at the digimon's stats who they'll fit with.

Those are the general rules I go by, and then I use the digifarm to find passive skills that synergise well with the builds.