Special, as always, is right.
First check the lease. When was it last renewed and who prepared it. Hopefully a decent agent and within the last year.
Properties are valuable, so the biggest risk is losing it, or not being able to get rid of a non-paying or neglectful tenant.
If it is not a recent assured short hold tenancy, and you don't know what it is, I would be tempted to have a lawyer review it and confirm that you are protected. If not, speak very nicely to the tenants and ask them to sign a new agreement. (If it is an old old tenancy they will have all sorts of rights and the lawyer may advise you that your only option is to sit tight or buy them out - but it would have to be a very old tenancy, or the last agreement signed would have to be inadequate .)
After that go through Specials' list.
Remember the property is your tenants home. Respect this. If things have not been well run, you may find they have a long list of valid complaints. Address issues as promptly as you can....your priority will be to have a new and suitable lease if you don't have one already.
It is always easier if you get on with the tenant, and this happens by being polite and reasonable. Anything in writing, however well you get on with them, should be with the thought that one day you might have to provide it. Most landlords and tenants (surprisingly) are reasonable people.
Do some research about rent levels with local agents. If you find an agent who sounds sensible, consider appointing them to manage. They should be able to sort out the agreement. (My experience is that larger agents are better at the legal stuff - economies of scale.) However I have generally been underwhelmed by agency management. Do not attempt to put the rent up until you have sorted out the lease.