A really useful tool for determining potential relationships is the Shared cM Project, along with the probabilities of those relationships. If you enter 1554 cM into the box, you get the following suggested relationships:
- 100%: Grandparent; Aunt/Uncle; Half-Sibling; Niece/Nephew; Grandchild
- 0%: Great-Aunt/Uncle; Great-Niece/Nephew (this set of relationships is just within the threshold for 1554cM, but has a zero probability in thednageek's table of probabilities)
This match is almost certainly not "just" a first cousin. One of the things that the Shared cM project doesn't account for is a double first cousin, which is what happens when two brothers marry two sisters and have children (the children are first cousins through each parent). Double first cousins have much higher shared cM, and could potentially appear as a half-sibling.
You should be able to use age to determine if grandparent/grandchild relationships are possible. For uncle, half-sibling, or nephew, age is a much less reliable indicator for what the potential relationship is, as there are people who have aunts/uncles within the same age group, or younger, nieces/nephews who are older, and half-siblings that are old enough to be their parent/young enough to be their child.