Buying a property combined with becoming a landlord requires a significant commitment, so make sure you've considered whether it's suitable for you. Research the buy-to-let market and make sure you're buying at the right time and in the right place. Do your sums to work out if you can afford the deposit and if the expected rental yield is worth it. If you're renting an existing property with a mortgage, you should obtain the lender's consent to the letting and you may need to change your mortgage product to a 'buy-to-let' mortgage. Also contact your building insurers to tell them that you are letting out the property.
Once you've decided to go down the buy-to-let route, have a think about the best type of property and location having thought about the sort of tenant you will be targeting (eg student, young professional or family). Spend some time selecting the most appropriate mortgage and make sure you carry out a survey on the property. If you're buying a property which is already rented out to a tenant, check that all the relevant documents are in place and that the deposit will be transferred to you. Also remember that there will be more restrictions when it comes to leasehold as opposed to freehold property.
For further information on buy-to-let read What to consider when buying a property to rent out, Points to consider before you buy-to-let and Buy-to-let as an investment and Buy to let as an investment in Scotland.