I've been doing it for a while so I can give you some things to consider. Never buy a condo as an investment property unless the margins are great. You can get killed with assessments. Buy single family homes with no HOA dues. Buy the smallest house you can buy in an area with high rent demand. People generally would rather have a small house than a small apartment. As others have said, buy in an area you know very, very well and as close to home as possible. Find a painter, handy man, HVAC, and plumber that you trust. Use a property management company so that they deal with the hassles, take care of the legal documents, and get tenants in the property. Don't ever buy a rental property with a pool. Don't buy properties with garages if you don't have to (just more maintenance). Also, if the tenants are handy, let them paint, fix stuff, etc. Just give them a break on rent. Keeping people in the properties long term avoids leasing costs, and long term tenants take care of the place like it is their house. You also have to be very picky and wait until you can buy under the market. Treat the investment as a bond and understand that the value is tied to net rents and not what other properties would sell for.
I spent a total of $203,000 on my three properties combined and put about $20,000 of improvements into them. I owe nothing on them and get $2550 in gross rents a month and yield about $1800-2000 per month after the property management fee, insurance, taxes, and maintenance (grass cutting, HVAC maintenance, and stuff like fixing appliances.
One last suggestion. Get in touch with a property manager. They will know if any investors what to sell, and often you can buy a property with a tenant in it.