100% uptime is a myth. What you can get is 100% uptime excluding planned maintenance, natural disasters etc.
To minimize the chances of having a downtime you need to make sure you have all of your data backed up regularly, so when yo need to migrate to a different server you can do it as swiftly as possible.
If you run an in-house dedicated server make sure only authorized professional personal have access physically and also logically (administrative remote access) to avoid any unnecessary accidents.
Make sure you have uninterruptible power supply in case of an power outage - backup generators etc.
However if you are renting a server in an modern datacenter most of them meet all of the security conditions. There is standardized methology used to clasify the level of security data centers have. They are called Tiers - They range from Tier 1 to Tier 4.
Most of the Data centers to day are TIER 3+ meaning they have redundant capacity components, dual-powered equipments and multiple uplinks, all components are fully fault-tolerant including uplinks, storage, chillers, HVAC systems, servers etc. everything is dual-powered.
However downtime is something that will always exist, what is more important is that the server provider has a good 24x7 support so it get the job done as fast as possible and most importantly that they let you know in advance that there will be a downtime. Often the downtime is only between 10-30 minutes to make necessary hardware changes. Not a big deal if you know about it but can make you angry if they turn of your service without notice.
