Mortgage help. First time buyer?

My partner and my self views a house a few days ago we want to out an offer in but we are first time buyers so don't no how this gos.
I have contacted the agency just waiting to here back. We had a meeting with a different agency month ago and he gave us an estimate of what mortgage we could get based on our wage. But how do I do this? Can I out am offer in then start the mortgage process? What do they do and how dose this work

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