Good form is key. I never used a trainer, but there are a lot of videos on youtube that outline common pitfalls and there are places like reddit's formcheck where people can critique your lifts if you upload a video.

Starting with low weights and working on form for the first week or two while video'ing yourself and comparing yourself to instructional videos will allow for self critique.

I personally never had a trainer, but have had good success with self critiquing.